THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

PRESENTED  BY 

PROF.  CHARLES  A.  KOFOID  AND 
MRS.  PRUDENCE  W.  KOFOID 

ASTRONOMY 
LIBRARY 


Essays  in  Astronomy 
by  Great  Astronomers 


The  World's  Great  Books 


Committee  of  Selection 
Thomas  B.  Reed  William  R.  Harper 

Speaker  of  the  House  President  of  the 

of  Representatives  University  of  Chicago 

Edward  Everett  Hale  Ainsworth  R.  Spofford 

Author  of  The  Man  Of  the  Congressional 

Without  a  Country  Library 

Rossiter  Johnson 

Editor  of  Little  Classics  and  Editor-in-Chief  of  this  Series 


Edition  de  Grand  Luxe 


Go-uplijr 


Essays  in  Astronomy 


B 


Ball,  Harkness,  Herschel,  Huggins, 

Laplace,  Mitchel,  Proctor, 

Schiaparelli,  and  Others 


i  a  Critical  Introduction  by 
Edward  Singleton  Holden 


i'WjbjnrfuoH  doDJs^  vd  gnivsTgns  tin  moH  3iir/jn§ 
i^I  vailboO  ii2  vd    nbaijs    £ 


D.  Appleto 


Photogravure  from  an  engraving  by  Jacob  Houbracken  after 
a  painting  by  Sir  Godfrey  Kneller. 


Essays  in  Astronomy 

By 

Ball,  Harkness,  Herschel,  Huggins, 

Laplace,  Mitchel,  Proctor, 

Schiaparelli,  and  Others 


With  a  Critical  Introduction  by 
Edward  Singleton  Holden 


Illustrated 


New  York 
D.  Appleton  and  Company 

1900 


COPYRIGHT,  1900, 
BY  D.  APPLETON  AND  COMPANY. 


QB3 


ASTRONOMf 

LIBRARY 


ESSAYS  ON   ASTRONOMY 


{T  is  the  design  of  the  series  of  which  this  volume  is  a 
part  to  give  in  complete  form  the  greatest  masterpieces 
in  every  department  of  literature,  and  in  science,  so  far 
as  practicable.  In  natural  science  the  object  may  be  at- 
tained by  presenting,  in  their  entirety,  the  World's  Great 
Books.  The  case  is  not  the  same  for  the  more  exact  sci- 
ences. There  is  no  single  book  on  astronomy,  for  example, 
that  displays  the  amazing  achievements  of  the  past  century 
in  untechnical  form.  On  the  other  hand,  there  are  many 
scattered  addresses  and  memoirs  that  present  a  single  aspect 
of  the  advance  in  an  authoritative  and,  at  the  same  time, 
in  a  popular  manner.  A  volume  might  now  be  written  that 
would  set  forth  the  present  state  of  scientific  opinion  on 
vital  astronomical  questions  and  exhibit  the  history  of  the 
successive  steps  by  which  our  actual  vantage-ground  has 
been  reached.  A  single  volume  of  the  sort  would  have  the 
great  advantage  of  symmetrical  presentation.  The  whole 
of  astronomy  might  be  viewed  from  our  present  standpoint, 
and  its  different  parts  displayed  in  their  just  historical  pro- 
portions. But  there  is  no  such  book. 

It  has  seemed  that  the  special  ends  in  view  could  be 
attained  by  extracting  from  the  writings  of  the  astronomers 
of  the  past  and  present  generations — mostly  from  those  of 
acknowledged  masters  of  science — their  own  accounts  of 
great  discoveries  made  by  themselves,  or  their  luminous 
reviews  of  the  discoveries  of  others,  their  colleagues.  No 
account  of  the  nebular  hypothesis  of  Laplace  from  the 
hand  of  another  could  give  the  vital  essence  of  his  great 

iii 


iv  ESSAYS  ON  ASTRONOMY 

generalization  so  vividly  and  succinctly  as  his  own  note, 
first  printed  in  1796  and  subsequently  amended  in  its  de- 
tails until  it  took  its  final  shape.  Here  we  have  the  matured 
reflections  of  a  great  philosopher  precisely  in  the  form  in 
which  they  were  marshalled  in  his  own  mind.  His  problem 
is  to  account  for  the  present  state  of  the  solar  system — how 
it  came  to  be  that  which  it  is.  We  are  bound  to  preserve 
the  substance  of  his  thoughts.  Can  they  be  better  pre- 
sented than  in  his  very  words?  A  paraphrase  of  Laplace's 
original  would  be  as  inappropriate  as  a  prose  version  of  one 
of  Milton's  immortal  sonnets.  The  present  volume  gives 
Laplace's  note  in  full. 

Again,  it  would  not  be  difficult  to  write  out  afresh  a 
satisfactory  account  of  the  achievements  of  the  spectro- 
scope in  determining  the  motions  and  the  physical  constitu- 
tions of  the  sun,  the  stars,  the  comets,  and  the  nebulae.  It 
was  a  novel  and  little  understood  instrument  when  Dr. 
Huggins  (now  Sir  William  Huggins)  first  devoted  it  to 
researches  on  the  heavenly  bodies.  In  1866,  in  the  first 
flush  of  successful  investigation,  Dr.  Huggins  paused  to 
make  a  report  on  the  extraordinary  results  that  he  had 
obtained  from  the  application  of  spectrum  analysis  to  the 
heavens.  Again,  in  1891,  a  quarter  of  a  century  later,  the 
veteran  observer  presented  to  the  British  Association  for 
the  Advancement  of  Science,  at  Cardiff,  an  elaborate  ac- 
count of  the  achievements  of  spectroscopists  up  to  that 
time.  Finally,  in  1897,  in  a  paper  on  the  "  New  Astron- 
omy," Sir  William  takes  a  retrospective  view  of  the  whole 
subject.  All  these  papers  from  the  same  skilled  hand  are 
printed  in  this  volume.  Taken  together,  they  exhibit,  as  no 
other  writings  can,  the  history  of  the  immense  advances 
that  have  been  made  in  this  novel  department  of  astro- 
nomical research.  The  earlier  paper  sets  forth  the  first 
results,  and  shows  what  was  even  then,  a  generation  ago, 
to  be  looked  for  in  the  future.  The  later  papers  review  the 
progress  that  has  actually  been  made,  and  indicate  the  re- 
sults that  are  safely  ours,  and  the  further  advances  that 
may  be  confidently  expected.  Here  we  have  what  we 


ESSAYS  ON  ASTRONOMY  V 

are  seeking  presented  to  us  by  a  master  hand,  and  we  be- 
come, in  some  sense,  witnesses  of  the  processes  of  research, 
and  appreciate  the  historical  relations  of  its  different  parts. 

It  is  to  be  noted  that  all  the  papers  reprinted  in  this 
volume  are  given  in  their  original  forms  and  in  their  en- 
tirety (except  those  of  the  present  writer,  which  have  been 
condensed,  and  except  that  a  single  paragraph  of  Sir  John 
Herschel's  has  been  added  to  supplement  an  address  that 
he  delivered  in  1841,  which  is  here  printed  in  full).  Sub- 
stance and  form  are  both  scrupulously  preserved. 

The  successive  chapters  in  this  book  have  the  great  ad- 
vantage, then,  of  setting  forth  the  state  of  the  questions  of 
which  they  treat  exactly  as  they  were  conceived  at  the  time 
of  writing  by  those  best  qualified  to  report  upon  them.  In 
general,  they  accurately  represent  the  present  state  of  opin- 
ion, or,  at  least,  they  are  suited  to  lead  up  to  it,  and  to 
exhibit  the  historic  development  of  the  science. 

This  series  of  chapters  is  by  no  means  addressed  to  pro- 
fessional astronomers,  although  it  collects  into  a  single  vol- 
ume many  papers  that  astronomers  will  be  glad  to  possess. 
On  the  contrary,  the  selections  have  been  made  especially 
to  meet  the  wants  of  the  general  reader,  and  to  furnish  him 
with  the  best  judgment  on  those  questions  of  astronomy 
that  appeal  to  us  all:  What  is  to  be  the  future  of  our  sys- 
tem? By  what  processes  of  evolution  has  it  come  to  be 
what  it  is?  How  long  may  we  expect  the  sun's  heat  to 
endure?  What  is  the  actual  physical  condition  of  the 
planets?  Are  they  suited  to  maintain  human  life?  Along 
with  these  fundamental  questions  others,  less  far-reaching, 
but  not  less  interesting,  are  treated. 

The  dimensions  of  the  solar  system  are  discussed  in  an 
address  delivered  before  the  American  Association  for  the 
Advancement  of  Science  in  1894  by  its  president,  Prof. 
William  Harkness,  Director  of  the  United  States  Naval 
Observatory  at  Washington.  The  results  of  the  great 
mathematical  astronomers — Lagrange,  Laplace,  and  others 
— upon  the  question  of  the  mechanical  stability  of  our 
system  are  set  forth  in  a  chapter  by  Mitchel,  founder  of 


vi  ESSAYS  ON   ASTRONOMY 

the  Cincinnati  Observatory.  The  lectures  and  addresses  of 
this  remarkable  man  had  the  greatest  influence  in  creating 
the  universal  and  intelligent  interest  in  astronomy  that  is 
so  distinctive  a  note  in  our  country.  His  lectures  were 
attended  by  thousands,  and  are  still  remembered.  The  Cin- 
cinnati Observatory  was  built  and  equipped  by  voluntary 
contributions  from  his  hearers,  and  it  is  largely  to  the  im- 
petus given  by  him  that  we  owe  the  establishment  of  so 
many  college  and  university  observatories  in  the  twenty 
years  preceding  our  civil  war.  The  chapter  here  repro- 
duced from  his  addresses  is  an  excellent  example  of  his  elo- 
quent manner  of  presenting  subjects  difficult  in  themselves 
and  foreign  to  ordinary  thinking. 

The  papers  upon  "  Atoms  and  Sunbeams,"  by  Sir  Rob- 
ert Stawell  Ball,  formerly  Astronomer-Royal  for  Ireland, 
and  now  Professor  of  Astronomy  in  the  University  of  Cam- 
bridge, England,  and  upon  the  "  Age  of  the  Sun's  Heat," 
by  Sir  William  Thomson,  now  Lord  Kelvin,  professor  in 
the  University  of  Glasgow,  present  two  aspects  in  the  study 
of  the  central  ruler  of  our  system.  In  the  latter  chapter 
we  have  the  reasons  for  Lord  Kelvin's  conclusions  upon 
the  past  and  future  duration  of  the  sun,  and  of  the  solar 
system  and  of  our  earth.  The  sun  can  not  have  ex- 
isted in  the  past  more  than  fifteen  million  or  twenty  million 
years,  nor  can  its  heat  support  life  on  the  globe  more  than 
ten  million  years  in  the  future. 

The  immense  periods  of  past  time  called  for  by  the  ge- 
ologist, and  especially  by  the  biologist,  to  account  for  ob- 
served changes  in  strata  and  in  the  species  of  animals,  are 
thus  denied  to  them  by  the  mathematical  astronomer.  The 
history  of  the  earth  must  be  compressed  into  a  comparatively 
few  millions  of  years.  On  the  other  hand,  the  future  of  our 
planet,  so  far  as  it  depends  upon  the  solar  heat,  is  strictly 
limited.  There  is  no  escape  from  the  general  conclusions 
of  Lord  Kelvin's  paper.  The  number  of  hundreds  of  thou- 
sands of  years  during  which  the  sun  will  furnish  heat  to 
the  earth  and  to  its  populations  can  not  be  fixed  with  ex- 
actness. But  it  appears  to  be  certain  that  our  system  is  not 


ESSAYS  ON   ASTRONOMY  vii 

self-sustaining,  and  that  its  life  is  not  to  be  indefinitely  long. 
It  is  an  organism  that,  like  every  other  organism  known  to 
us,  must  come  to  an  end  in  consequence  of  the  very  laws  that 
now  keep  it  active.  It  must  have  had  a  beginning  within  a 
period  that  can  not  be  fixed  with  certainty,  but  which  can 
not,  in  any  event,  much  exceed  twenty  million  years;  and  it 
must  end,  so  far  as  life  is  concerned,  in  a  chaos  of  cold  and 
dead  globes  at  a  calculable  time  in  the  future,  when  the 
sun  has  radiated  away  its  store  of  heat ;  unless  that  heat  is  re- 
created by  the  operation  of  forces  now  totally  unknown  to  us. 

At  the  annual  meeting  of  the  ancient  Accademia  dei 
Lined  in  Rome  (Galileo's  Academy),  in  1889,  in  the  pres- 
ence of  the  tting  and  Queen  of  Italy,  the  famous  astrono- 
mer of  Milan — Schiaparelli — was  called  upon  to  report  on 
his  remarkable  conclusions  regarding  the  rotation  and  the 
physical  constitution  of  the  planet  Mercury.  His  paper  is 
printed  as  a  chapter  in  this  volume.  In  it  he  announced 
that  the  planet  Mercury  revolved  about  the  sun,  turning 
always  one  face  to  it,  as  the  moon  always  turns  one  face 
to  the  earth.  In  the  one  case  as  in  the  other,  the  efficient 
cause  has  been  the  action  of  tides,  which  operate  to  reduce 
the  rate  of  axial  rotation.  Working  always  in  one  direction 
— to  retard  and  never  to  accelerate — the  tides  may,  in  time, 
retard  the  motion  of  rotation  of  a  planet  (as  they  have  the 
motion  of  our  own  satellite)  until  it  presents  a  single  face 
to  its  central  body.  This  face  of  Mercury  alone  receives 
solar  light  and  heat;  the  other  is  condemned  to  perpetual 
darkness.  The  conditions  of  life  (if  so  be  that  life  exists) 
upon  such  a  planet  are  as  different  as  possible  from  those 
that  we  experience  upon  our  own  earth;  and  Schiaparelli 
has  lucidly  expounded  the  consequences  of  such  condi- 
tions as  exist  on  Mercury. 

As  early  as  1882  M.  Schiaparelli  confided  to  one  of  his 
correspondents  his  discovery  in  the  verses  that  follow: 

"  Cynthiae  ad  exemplum  versus  Cyllenius  axe 
Sternum  noctem  sustinet,  atque  diem: 
Altera  perpetuo  facies  comburitur  sestu, 
Abdita  pars  tenebris  altera  Sole  caret." 


viii  ESSAYS  ON   ASTRONOMY 

A  little  later  he  announced,  after  a  comparison  between 
observations  made  at  Milan,  Washington,  and  Brussels, 
that  the  planet  Venus  was  in  like  case.  Venus,  too,  revolves 
like  our  moon,  turning  once  on  its  axis  during  one  revolu- 
tion about  its  central  body  (in  225  days  for  Venus,  88  days 
for  Mercury,  and  29  days  for  the  moon). 

The  observations  of  Mr.  Percival  Lowell,  in  Arizona, 
seem  to  confirm  Schiaparelli's  conclusions.  There  is  noth- 
ing in  the  observations  made  at  the  Lick  Observatory  to 
throw  doubt  upon  them.  The  question  is  very  difficult.  If 
the  results  of  M.  Schiaparelli  are  finally  conclusively  estab- 
lished, we  shall  have  an  entirely  novel  set  of  circumstances 
in  the  family  of  planets.  A  mathematical  investigation  of 
the  effects  of  tidal  retardation  in  the  case  of  the  moon  shows 
that  certain  effects  are  possible,  and  that  the  rotation  of  a 
satellite  to  the  earth  (the  moon),  or  to  the  sun  (a  planet), 
may  go  through  strange  alterations  during  its  life  history. 
Not  only  may  the  period  of  rotation  be  changed,  but  the 
distance  from  the  central  body  may  vary  enormously.  Our 
moon  now  turns  a  single  face  to  the  earth.  Schiaparelli 
interprets  his  observations  as  showing  that  two  out  of  the 
eight  planets  of  our  system  are  likewise  in  this  stage  of 
evolution.  These  and  other  planets  will  in  the  future  pass 
through  very  various  circumstances. 

A  chapter  is  also  devoted  to  the  presentation  of  Pro- 
fessor Schiaparelli's  conclusions  as  to  the  planet  Mars. 
The  views  of  Schiaparelli,  who  is  an  observer  of  the  highest 
competence,  and  a  skilled  mathematician  and  physicist, 
must  not  be  confounded  with  the  reports  of  his  work  that 
have  been  made  from  time  to  time  by  sanguine  popular 
writers  who  have  decided  a  priori  that  the  Universe  was 
created  to  be  peopled  by  human  beings,  and  who  have  dis- 
torted Schiaparelli's  guarded  and  cautious  words  into  defi- 
nite pronouncements  that  Mars,  at  least,  is  inhabited  by 
a  race  of  high  intelligence.  Nor  does  the  mass  of  writ- 
ing in  newspapers  and  ephemeral  publications  upon  the 
question  of  life  in  Mars  deserve  any  scientific  consideration 
whatever.  Weight  of  a  certain  kind  it  has.  There  is  no 


ESSAYS  ON   ASTRONOMY  il 

doubt  that  thousands  of  readers  get,  and  must  get,  their 
acquaintance  with  scientific  conclusions  from  newspaper 
paragraphs  compiled  at  third  and  fourth  hand  by  persons 
without  accurate  and  sufficient  technical  knowledge.  Many 
such  persons  are  now  fully  satisfied  that  somehow  and  some- 
where, some  one,  probably  Schiaparelli,  has  decided  in  a 
scientific  manner,  first,  that  Mars  is  a  planet  very  much 
like  our  own  earth,  with  a  sufficient  atmosphere,  abundant 
water,  and  all  the  conditions  for  habitability;  second,  that 
the  "  canals  "  on  the  planet  are  without  doubt  the  work 
of  human  hands,  and  a  convincing  proof  of  intelligent  en- 
gineering; and,  third,  that  we  are  now  on  the  point  of 
communicating  by  some  kind  of  electric  signalling  with 
our  cosmic  brothers.  An  ardent  desire  that  all  this  should 
be  true  is  taken  as  convincing  proof  that  it  has  veritably 
been  proved. 

The  judgment  of  the  majority  of  competent  astrono- 
mers on  these  points  is  very  different.  They  know  how 
very  far  we  still  are  from  a  determination  of  the  question 
whether  Mars,  or  any  other  planet,  is,  in  fact,  inhabited. 
They  know  that  this  question,  universally  interesting  as  it 
is,  by  no  means  represents  the  present  inquiry  of  science, 
which  is:  Are  any  of  the  planets  habitable?  Are  they  fit 
to  be  inhabited?  Even  this  limited  inquiry  is  still  so  far 
from  an  answer  that  it  is  almost  idle  to  speculate  on  the 
larger  question.  Unless  it  can  be  proved — as  it  certainly 
has  not  been — that  Mars  is  a  planet  where  conditions  pre- 
vail that  are  favourable  to  human  existence,  it  is  a  simple 
waste  of  effort  to  inquire  whether  such  life  is  now  in  exist- 
ence. No  one  can  say  with  any  kind  of  scientific  certainty 
whether  a  human  being  transported  to  Mars  could  live 
even  for  an  instant.  It  is  more  than  probable  that  he 
could  not. 

On  account  of  the  intense  interest  that  such  questions 
excite,  a  few  remarks  are  here  devoted  to  a  review  of 
the  case  as  it  stands.  In  the  first  place,  we  may  safely  say 
that  Mars  has  little  or  no  atmosphere,  basing  this  conclu- 
sion, at  the  outset,  upon  its  capacity  of  reflecting  the  solar 


X  ESSAYS   ON   ASTRONOMY 

light  that  falls  upon  its  surface,  as  compared  with  the  like 
capacity  of  other  bodies  of  the  solar  system.  The  moon 
reflects  ^V  of  the  light  falling  upon  it — about  as  much  as 
sandstone  rocks.  Mercury  reflects  -^j-.  These  bodies  have 
little  or  no  atmosphere.  Venus  reflects  (from  the  outer  sur- 
face of  its  envelope  of  clouds)  -ffo  of  the  incident  light. 
Jupiter  d%),  Saturn  (ffr),  Uranus  (TVir)>  Neptune  (rW)> 
are  all  surrounded  by  extensive  atmospheres  and  all  have 
high  reflecting  powers.  The  corresponding  number  for 
Mars  (TI&-)  is  so  small  as  to  indicate  that  this  planet  has 
little  atmosphere,  if  any.  Again,  the  planet's  surface  has 
been  under  careful  scrutiny  for  many  years,  and  observers 
are  all  but  unanimous  in  their  report  that  no  clouds  are 
visible  over  the  surface.  The  centre  of  the  disks  of  bodies 
with  extensive  atmospheres  (the  sun,  Jupiter,  Saturn, 
etc.)  is  always  brighter  than  the  edges.  The  centre  of 
the  moon,  which  has  no  atmosphere,  is  not  so  bright  as 
the  edge.  Mars  is  like  the  moon  in  this  respect,  and 
not  like  Jupiter.  Finally,  the  only  satisfactory  spectro- 
scopic  observations  of  the  planet  (made  independently 
at  the  Lick  Observatory  and  at  the  Allegheny  Observa- 
tory) show  no  evidence  whatever  of  an  atmosphere  to 
Mars  and  no  sign  of  water-vapour  about  the  planet. 
If  there  is  any  atmosphere  at  all,  it  can  hardly  be  more 
dense  than  the  earth's  atmosphere  at  the  high  summits 
of  the  Himalaya  Mountains — not  enough  to  support 
human  life,  therefore.  As  there  is  no  evidence  of  the  pres- 
ence of  water-vapour  and  of  clouds,  etc.,  it  follows  that  there 
is  little  or  no  water  on  the  planet's  surface.  The  spectrum 
of  Mars  and  the  spectrum  of  the  moon  are  identical  in  every 
respect.  This  could  not  be  true  if  Mars  had  any  consid- 
erable atmosphere. 

The  important  and  long-continued  observations  of 
Schiaparelli  on  Mars  led  him  to  announce  that  the  planet 
was  provided  with  an  elaborate  system  of  water-courses 
("  oceans,  seas,  lakes,  canals,  etc."),  and  the  authority  of 
this  distinguished  observer  is  the  chief  support  of  those  who 
maintain  that  the  planet  is  fit  for  human  habitation. 


ESSAYS  ON  ASTRONOMY  xi 

Complete  explanations  of  all  the  phenomena  presented 
by  the  planet  can  not  be  given  in  the  light  of  our  present 
knowledge.  This  is  not  to  be  wondered  at,  in  spite  of 
the  industry  and  ability  of  the  observers  who  have  spent 
years  in  studying  its  surface.  The  case  is  much  the  same 
for  the  planets  Mercury,  Venus,  Jupiter,  Saturn,  Uranus, 
Neptune.  We  know  very  little  of  the  real  conditions  that 
prevail  on  their  surfaces.  We  know  comparatively  little 
of  the  interior  of  the  earth  on  which  we  live,  and  next  to 
nothing  about  the  interior  of  other  planets.  There  is  every 
reason  to  believe  that  complete  explanations  will  be  forth- 
coming in  time.  It  is,  at  any  rate,  certain  that  the  conclu- 
sions of  Schiaparelli,  named  above,  can  not  be  accepted 
without  serious  modification.  In  the  telescope  the  main 
body  of  the  planet  is  reddish,  and  there  are  many  perma- 
nent dark  markings  of  a  grayish-blue  colour.  The  polar 
caps  are  sometimes  dazzlingly  white. 

When  Sir  William  Herschel  was  examining  Mars  in 
the  eighteenth  century  he  called  the  red  areas  of  Mars 
"  land  "  and  the  greenish  and  bluish  areas  "  water."  It 
was  a  general  opinion  in  his  day  that  all  the  planets  were 
created  to  be  useful  to  man.  Astronomers  of  the  eighteenth 
century  set  out  with  this  belief  very  much  as  the  phi- 
losophers of  Ptolemy's  time  set  out  with  the  fundamental 
theorem  that  the  earth  was  the  centre  of  the  motions  of  the 
planets.  For  example,  Herschel  maintained  that  the  sun 
was  cool  and  habitable  underneath  its  envelope  of  fire.  He 
says  (1795),  "The  sun  appears  to  be  nothing  else  than  a 
very  eminent,  large,  and  lucid  planet,  most  probably  also 
inhabited  by  beings  whose  organs  are  adapted  to  the  pe- 
culiar circumstances  of  that  vast  globe."  It  is  certain 
that  the  sun  is  not  inhabited  by  any  beings  with  organs. 
This  conclusion  is  now  as  obvious  as  that  no  beings  inhabit 
the  carbons  of  an  electric  street  lamp.  Herschel's  guess 
that  the  red  areas  on  Mars  were  land  and  the  blue  areas 
water  had  no  more  foundation  than  his  guess  that  the  sun 
might  be  inhabited. 

The  next  careful  studies  of  Mars  were  made  by  Maedler 


xii  ESSAYS   ON   ASTRONOMY 

about  1840.  He  also  called  the  red  areas  of  the  disk  land 
and  the  dark  areas  water.  In  this  he  followed  Herschel. 
There  was  no  reason  why  he  should  not  have  called  the 
red  areas  water  and  the  dark  areas  land.  He  had  absolutely 
no  evidence  on  the  point.  The  same  is  true  of  later  ob- 
servers down  to  the  first  observations  of  Schiaparelli  about 
1877.  Schiaparelli  gave  reasons  for  these  names,  though 
his  reasons  are  not  convincing.  He  pointed  out  that  the 
narrow  dark  streaks  ("  canals  ")  generally  ended  in  large 
dark  areas  ("  oceans  ")  or  in  smaller  dark  areas  ("  lakes  "). 
The  narrow,  dark  streaks  (very  seldom  less  than  sixty  miles 
wide)  are  quite  straight.  They  can  not  be  rivers,  then.  If 
they  are  water  at  all,  the  name  "  canal "  is  not  inappro- 
priate, though  sixty  or  one  hundred  miles  is  a  very  wide 
canal.  If  they  are  water,  then  the  large  dark  areas  must  be 
seas.  The  narrow,  dark  streaks  are  not  water,  however, 
because  it  was  discovered  by  Dr.  Schaeberle  at  the  Lick 
Observatory  that  the  so-called  "  seas  "  sometimes  had  so- 
called  "  canals  "  crossing  them.  A  sea  traversed  by  a  canal 
is  an  absurdity.  It  is  maintained  by  a  few  recent  observers 
of  Mars  that  some  of  the  dark  areas  are  water,  and  some 
are  not  so.  The  bluish-green  colour  of  the  dark  spots  is 
said  to  "  suggest  vegetation."  But  who  can  know  what 
colours  the  vegetation  on  Mars  may  have? 

The  foregoing  very  brief  abstract  of  a  long  history 
proves  that  the  dark  areas  on  Mars  are  not  water.  The  red 
areas  are  not  known  to  be  land.  The  spectroscopic  and 
other  evidence  proves  that  Mars  has  little  or  no  atmosphere 
— little  or  no  water-vapour — no  clouds.  It  is  not  yet  known 
what  is  the  real  nature  of  the  red  areas  and  of  the  dark  areas. 
It  is  one  of  the  many  unsolved  problems  of  astronomy  to 
discover  the  answer  to  this  fundamental  question.  There 
is  no  doubt  that  the  red  areas  and  the  large  dark  areas  have 
a  real  existence,  since  some  of  the  markings  on  Mars  have 
been  seen  for  more  than  two  centuries. 

It  is  not  certain  that  all  the  "  canals  "  that  have  been 
mapped  really  exist.  Some  of  them  are  probably  mere 
optical  illusions.  If  they  were  real  streaks  on  the  planet's 


ESSAYS  ON   ASTRONOMY  xiii 

surface  (like  wide  fissures,  broad  water-courses,  etc.)  they 
would  always  appear  broadest  when  they  were  at  the  centre 
of  the  disk  and  would  always  be  narrower  when  they  were 
at  the  edges.  The  laws  of  perspective  demand  this.  It  is 
found  by  observation  that  the  reverse  is  frequently  true. 

The  distance  of  Mars  from  the  sun  is  one  and  a  half 
times  the  earth's  distance.  The  heat  received  by  the  earth 
from  the  sun  is  to  the  heat  received  by  Mars  as  (1.5)2  = 
2.25  to  i.  Mars  receives  less  than  half  as  much  sun  heat 
as  the  earth.  If  the  earth  had  no  more  atmosphere  than 
the  moon,  the  earth's  temperature  would  be  like  that  of  the 
moon.  If  the  earth  had  no  denser  atmosphere  than  that  on 
the  summits  of  the  Himalayas,  the  temperature  of  the  earth 
would  always  be  below  zero.  Human  life  could  not  exist 
here.  The  case  is  the  same  with  Mars.  The  temperature 
of  the  whole  surface  of  the  planet  must  be  extremely  low, 
even  in  its  equatorial  regions.  The  temperature  at  the 
poles  of  Mars  must  be  several  hundred  degrees  (Fahren- 
heit) below  zero  when  the  pole  is  turned  away  from  the 
sun,  and  below  zero  even  when  the  pole  is  turned  toward 
the  sun. 

Before  going  further  it  is  worth  while  to  consider  the 
circumstances  under  which  Mars  is  seen  by  an  observer  on 
the  earth.  The  mean  distance  of  the  moon  from  the  earth 
is  240,000  miles.  If  it  is  viewed  through  a  field  glass  mag- 
nifying four  times,  it  is  virtually  brought  within  60,000 
miles  of  the  observer.  The  nearest  approach  of  Mars  to 
the  earth  is  35,000,000  miles.  The  planet  can  very  seldom 
be  viewed  to  advantage  with  a  magnifying  power  so  high 
as  five  hundred.  If  such  a  power  is  employed  when  Mars 
is  nearest,  the  planet  is  virtually  brought  within  70,000 
miles.  It  follows,  therefore,  that  we  never  see  Mars  so  ad- 
vantageously even  with  the  largest  telescopes  as  we  may 
see  the  moon  in  a  common  field  glass.  If  the  reader  will 
examine  the  moon  with  a  field  glass  magnifying  four  times, 
he  will  have  a  realizing  sense  of  the  best  conditions  under 
which  it  is  possible  to  see  Mars,  and  he  will  be  surprised 
that  so  much  is  known  of  the  planet.  The  industry  and 


xiv  ESSAYS   ON   ASTRONOMY 

fidelity  of  observers  can  only  be  appreciated  after  such  an 
experiment. 

Observations  upon  the  polar  caps  of  Mars  must  be 
interpreted  in  the  light  of  the  foregoing  facts — namely,  that 
Mars  has  little  or  no  water-vapour,  and  that  its  temperature 
is  appallingly  low.  The  main  facts  of  observation  are  as 
follows:  Cassini,  the  royal  astronomer  of  France,  discov- 
ered in  1666  that  Mars  sometimes  had  dazzling  white  cir- 
cular patches  near  his  poles.  In  1783  Sir  William  Herschel 
observed  these  patches  to  wax  and  wane,  and  he  called 
them  "  snow  "  caps,  thus  begging  the  question  as  to  their 
real  nature.  Herschel's  observations  and  those  of  all  later 
observers  show  that  these  caps  wax  and  wane  with  the  Mar- 
tian seasons.  In  the  Martian  polar  summer  they  are  small- 
est, or  they  even  vanish;  in  the  Martian  polar  winter  they 
are  largest.  As  Herschel  began  with  the  conviction  that  all 
planets  were  analogous  to  the  earth  and  were  meant  to  be 
inhabited,  his  conclusion  was  that  the  polar  winter  con- 
densed water-vapour  into  snow,  and  that  the  polar  summer 
melted  this  snow,  and  so  on.  A  more  scientific  conclusion 
would  have  been  that  some  vapour  was  condensed  and  sub- 
sequently dissipated  by  the  solar  heat.  It  is  practically 
certain  that  the  phenomena  of  the  waxing  and  waning  of 
the  caps  depend  on  solar  heat. 

If  the  caps  are  snow  condensed  from  water-vapour,  the 
layer  of  snow  must  be  exceedingly  thin,  because  when  these 
caps  are  melted  no  clouds  appear.  When  snow  melts  on 
the  earth,  clouds  are  formed  and  our  atmosphere  is  charged 
with  the  vapour  of  water.  No  clouds  are  seen  on  Mars, 
and  no  water-vapour  is  to  be  found  above  its  surface  by  any 
spectroscopic  test. 

The  polar  caps  must  be  formed  by  the  vapour  of  some 
other  substance  than  water.  It  is  worth  while  to  inquire 
whether  they  may  not  be  carbon  dioxide  in  a  solid  state. 
This  substance  is  a  heavy  gas  (carbonic-acid  gas)  at  ordi- 
nary temperatures.  It  would  lie  at  the  bottom  of  valleys 
and  fill  canons  or  ravines.  At  a  temperature  of  about  one 
hundred  Fahrenheit  below  zero  it  is  a  colourless  liquid. 


ESSAYS   ON   ASTRONOMY  XV 

At  temperatures  such  as  must  obtain  at  the  pole  of  Mars 
turned  away  from  the  sun  it  becomes  a  snowlike  solid. 
Caps  of  carbon  dioxide  would  wax  and  wane  at  the  poles  of 
Mars  under  variations  of  solar  heat  such  as  obtain  at  these 
poles,  very  much  as  caps  of  snow  and  ice  wax  and  wane  in 
our  arctic  regions,  which,  under  all  circumstances,  are  at 
a  higher  temperature  than  the  poles  of  Mars. 

There  is  so  far  no  observational  proof  that  the  polar 
caps  of  Mars  are  formed  of  carbon  dioxide.  There  is  con- 
vincing proof  that  they  are  not  formed  of  water.  The  ques- 
tion as  to  the  nature  of  the  polar  caps  is  still  open. 

The  question  of  life  in  other  worlds  is  vitally  and  pro- 
foundly interesting  to  all  mankind,  and  it  is  discussed  in 
several  chapters  of  this  volume.  But  the  inquiries  of  sci- 
ence must  at  present  be  limited  to  a  determination  of  more 
humble  questions.  Are  the  planets  habitable?  is  the 
present  problem;  later,  we  may  perhaps  be  able  to  attack 
the  question  whether  they  are,  in  fact,  inhabited. 

A  chapter  on  "  Sidereal  Astronomy,  Old  and  New," 
gives  an  account  of  researches  that  led  to  an  inventory  of  the 
stellar  universe,  and  of  some  of  the  main  results  attained, 
and  serves  as  an  introduction  to  two  memorable  addresses 
by  Sir  John  Herschel  on  the  occasion  of  awarding  the 
gold  medal  of  the  Royal  Astronomical  Society  to  Bessel 
and  to  Francis  Baily  for  their  great  catalogues  of  stars. 
The  true  significance  of  the  work  of  the  observing  astrono- 
mer is  not  grasped  until  it  is  understood  to  how  many 
and  various  uses  the  results  of  his  patient  and  laborious 
watchings  may  be  applied.  These  addresses  of  Herschel 
are,  moreover,  models  of  lucid  and  eloquent  exposition. 

Two  papers  on  the  "  Mathematical  Theories  of  the 
Earth,"  by  Professor  Robert  Simpson  Woodward,  of  Co- 
lumbia University,  and  on  the  "  Wanderings  of  the  North 
Pole,"  by  Sir  Robert  Ball,  exhibit  the  astronomical  rela- 
tions of  our  planet  in  an  unfamiliar  light,  and  describe  one 
of  the  capital  discoveries  of  modern  times — the  variability 
of  terrestrial  latitudes.  Incidentally,  they  serve  to  display 
the  vitality  of  the  "  old  "  exact  astronomy.  The  "  new  " 


xvi  ESSAYS   ON   ASTRONOMY 

astronomy,  which  avails  itself  of  photographic  and  spectro- 
scopic  aids,  appeals  more  directly  to  popular  imagination, 
and  its  results  can  be  presented  to  a  popular  audience 
through  a  series  of  striking  photographic  pictures,  but  it  is 
helpless  unless  it  employs  the  rigid  methods  of  its  elder 
sister  science;  and  new  triumphs  in  many  fields  await  the 
votary  of  the  old  astronomy.  The  recent  discovery  of 
variable  terrestrial  latitudes  is  not  the  least  of  them. 

The  papers  on  the  "  History  of  the  Astronomical  Tele- 
scope," by  Professor  Charles  Hastings,  of  Yale  University; 
on  "  An  Astronomer's  Life  in  a  Modern  Observatory,"  by 
Dr.  David  Gill,  royal  astronomer  at  the  Cape  of  Good 
Hope;  on  "  Photography  as  the  Servant  of  Astronomy," 
and  on  the  "  Beginnings  of  American  Astronomy,"  by  the 
writer,  present  the  history  of  astronomy  in  this  country 
and  elsewhere,  and  exhibit  certain  little-understood  aspects 
of  modern  astronomical  activity.  These  chapters  of  actu- 
alities will,  it  is  believed,  be  especially  welcome  to  readers 
who  are  familiar  with  the  results  of  astronomical  work,  but 
who  are  not  acquainted  with  its  details. 

Laplace's  Nebular  Hypothesis — a  household  word — 
was  first  proposed  in  1796.  He  mentioned  the  suggestions 
of  Buffon  with  respect.  It  is  an  interesting  fact  that  Buf- 
fon  possessed  a  copy  of  the  "  Principia  "  of  Swedenborg, 
in  which  the  Swedish  philosopher  had  expounded  (1734) 
a  hypothesis  to  account  for  the  genesis  of  the  solar  system. 
The  cosmologies  of  Swedenborg,  Buffon,  and  Laplace 
stand  in  a  kind  of  historic  relation  to  each  other.  The 
theory  propounded  by  Laplace  must,  of  course,  be  regarded 
as  an  independent  and  original  generalization  of  a  master 
mind.  His  obligations  to  his  predecessors  were  vanish- 
ingly  small. 

The  astronomical  question  that  touches  us  most  inti- 
mately and  deeply  is  that  of  life  in  other  worlds.  We  know 
our  own  life  here  in  its  thousand  forms.  We  see  the  firma- 
ment studded  with  stars,  each  star  a  sun,  and  each  sun  pre- 
sumably accompanied  by  its  retinue  of  planets.  Our  own 
sun,  a  fairly  insignificant  star,  has  eight  planets  in  its  train, 


ESSAYS  ON   ASTRONOMY  xvii 

and  one  of  them  we  know  to  swarm  with  life.  How  is 
it  with  the  planets  that  accompany  those  millions  of  other 
suns?  The  instructed  mind  refuses  to  accept  their  shining 
as  accessory  to  our  little  existence.  Man  was  the  king  and 
centre  of  the  universe.  The  telescope  discrowned  him. 
To  the  eye  of  reason  he  became  a  mere  ephemeral  being, 
living  out  a  short  and  troubled  life  on  the  surface  of  one  of 
the  smaller  bodies  of  a  vast  universe.  And  the  universe 
itself  is  limited.  We  have  seen  that  it  is  to  end  in  dark- 
ness, as  it  began.  The  immensity  of  man's  fall  in  dignity  has 
often  been  insisted  upon.  It  is  sometimes  forgotten  that, 
after  all,  it  was  by  a  man  that  man  was  discrowned.  If  he 
is  not  the  master  of  his  own  fate,  at  least  he  has  foreseen 
it.  Has  he  companions  of  like  nature  on  the  planets  of 
his  own  system,  or  in  the  worlds  that  accompany  each  of 
the  brilliant  stars  of  a  winter's  night?  The  subject  is  of 
immense  intellectual  interest,  and  it  is  treated  in  two  chap- 
ters of  this  volume:  by  the  late  Richard  Proctor,  on  a 
"  New  Theory  of  Life  in  Other  Worlds,"  and  by  the  Rev- 
erend Father  Searle,  some  time  Professor  of  Astronomy  in 
the  Catholic  University  of  America,  on  the  "  Habitability 
of  the  Planets." 

A  glance  over  the  table  of  contents  will  show  that  the 
present  volume  contains  essays  on  a  great  variety  of  sub- 
jects, written  at  first  hand,  mostly  by  the  great  astronomers. 
Place  has  been  found  for  chapters  on  the  sun,  the  planets, 
meteorites,  stellar  systems,  and  on  our  own  earth;  on  the 
magnitude  and  stability  of  the  solar  system;  on  the  nebular 
hypothesis;  on  sidereal  astronomy;  on  photography  and 
spectroscopy  in  their  astronomical  relations;  on  the  history 
of  American  astronomy,  and  of  the  telescope  in  all  lands;  on 
the  special  aims  and  methods  of  the  modern  astronomer; 
and,  finally,  on  the  theory  of  life  in  other  worlds  than  ours. 
Taken  together,  they  present  a  vivid  picture  of  the  present 
state  of  scientific  opinion,  and  they  afford  a  historical  per- 
spective that  is  of  high  value. 

EDWARD  S.  HOLDEN. 


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CONTENTS 

ATOMS  AND  SUNBEAMS  PAGK 

By  Sir  Robert  Stawell  Ball  .        .      ",        .        .        .        .        .        I 

THE  WANDERINGS  OF  THE  NORTH  POLE 

By  Sir  Robert  Stawell  Ball  .        .        .        .     '    .  "l  .        .'  '     .      22 

THE  AGE  OF  THE  SUN'S  HEAT 

By  Sir  William  Thomson  (Lord  Kelvin)      .        .        .        •.>    •      4* 

THE  PAST  AND  FUTURE  OF  OUR  EARTH 

By  Richard  Anthony  Proctor       .        .        9    ._,       -.,.«?.       55 

A  NEW  THEORY  OF  LIFE  IN  OTHER  WORLDS 

By  Richard  Anthony  Proctor       .        .        .        .        .        .        .83 

MATHEMATICAL  THEORIES  OF  THE  EARTH 

By  Robert  Simpson  Woodward  .        .        .        .        .    ,  t,     f-.     103 

THE    ROTATION    AND    PHYSICAL    CONSTITUTION    OF    THE 
PLANET  MERCURY 

By  Giovanni  Virginio  Schiaparelli      ..        ,        .'      .        .        .     131 
THE  PLANET  MARS 

By  Giovanni  Virginio  Schiaparelli        .        .        .        .        .        .     143 

METEORITES  AND  STELLAR  SYSTEMS 

By  George  Howard  Darwin         .......     163 

MAGNITUDE  OF  THE  SOLAR  SYSTEM 

By  William  Harkness  .        .        .        .        .        .        .        .        .183 

THE  STABILITY  OF  THE  SOLAR  SYSTEM 

By  Ormsby  McKnight  Mitchel    .        .        .        .        .        .        .213 

xix 


xx  ESSAYS  IN   ASTRONOMY 

THE  NEW  PLANET,  EROS  PAGE 

By  Edmund  Ledger     .        .        .        .        .        ,        •    ^   •        •     239 

SIDEREAL  ASTRONOMY  :  OLD  AND  NEW 

By  Edward  Singleton  Holden .253 

PHOTOGRAPHY  THE  SERVANT  OF  ASTRONOMY 

By  Edward  Singleton  Holden 290 

THE  BEGINNINGS  OF  AMERICAN  ASTRONOMY 

By  Edward  Singleton  Holden      .'*.*'''""' 309 

STELLAR  PARALLAX 

By  Sir  John  Frederick  William  Herschel     .        .        .    .    «        .     321 

THE  HISTORY  OF  THE  TELESCOPE 

By  Charles  S.  Hastings        .        .       '.,,»,.,    ...»        n        .     339 

RESULTS  OF  SPECTRUM   ANALYSIS  APPLIED  TO   HEAVENLY 

BODIES 
By  William  Huggins    .         .''       .        J.        '•''      •  J   "••"'**•        .     363 

CELESTIAL  SPECTROSCOPY 

By  William  Huggins    .        .        .        .  '     ,l  ?^r  "''*'       •        •     391 
THE  NEW  ASTRONOMY 

By  William  Huggins    .         .         .         .        /      .       V        .        .    441 

AN  ASTRONOMER'S  WORK  IN  A  MODERN  OBSERVATORY 

ByDavidGill      /  .        .        .      ''''.'    <;  .  •  ^.        .    473 

THE  SYSTEM  OF  THE  WORLD — THE  NEBULAR  HYPOTHESIS 

By  Pierre  Simon,  Marquis  de  Laplace         .        ...        .    497 

ARE  THE  PLANETS  HABITABLE? 

By  George  M.  Searle ..    '   ."   .     513 


ILLUSTRATIONS 

FACING  PAGE 

SIR  ISAAC  NEWTON       .       .       .  .       .         Frontispiece 

Photogravure  from  an  engraving  by  Jacob  Houbracken  after  a 
painting  by  Sir  Godfrey  Kneller 

WlLHELM  AND   LUDWIG,   DUKES  OF  BAVARIA    ....    XVlii 

Fac-simile  of  a  page  in  the  "  Law  Code"  folio,  printed  at  Ingol- 
stadt,  1516  A.  D. 

THE  NEBULA  IN  ORION       .       .       .       .       .       .       .       .      60 

Photogravure  from  a  photograph  taken  at  the  Lick  Observatory 

NICOLAS  COPERNICUS  , 188 

Photogravure  from  a  painting 

FIRST  OBSERVATION  OF  THE  TRANSIT  OF  VENUS    .       .        .    244 

Photogravure  from  an  old  etching 

THE  LICK  OBSERVATORY      .       .       .       .       .       ...    306 

Photogravure  from  a  photograph 

FRAUNHOFER  DEMONSTRATING  THE  SPECTROSCOPE  .     ...    446 
Photogravure  from  a  painting  by  Richard  Wimmer 

THE  MOON'S  SURFACE 518 

Photogravure  from  a  photograph  taken  at  the  Lick  Observatory 

xxi 


ATOMS  AND  SUNBEAMS 

AND 

THE  WANDERINGS  OF  THE 
NORTH  POLE 


BY 

SIR   ROBERT   STAWELL   BALL 


ATOMS  AND   SUNBEAMS1 

IN  recent  years  an  important  change  has  taken  place 
in  the  manner  in  which  many  physical  problems  are 
approached.  The  philosopher  who  now  seeks  an  ex- 
planation of  great  natural  phenomena  not  unfrequently 
finds  much  assistance  from  certain  remarkable  discoveries 
as  to  the  ultimate  constitution  of  matter.  Many  an  obscure 
question  in  physics  has  been  rendered  clear  when  some  of 
the  properties  of  molecules  have  been  brought  to  light. 
No  doubt  our  knowledge  of  the  natural  history  of  the 
molecule  is  still  vastly  wanting  in  detail.  It  must,  however, 
be  admitted  that  we  have  traced  an  outline  of  that  won- 
derful chapter  in  Nature  which  is  specially  serviceable  in 
the  question  which  I  now  propose  to  discuss. 

The  problem  before  us  may  be  stated  in  the  following 
terms:  We  have  to  illustrate  how  the  sun  is  enabled  to 
maintain  its  tremendous  expenditure  of  light  and  heat  with- 
out giving  any  signs  of  approaching  exhaustion.  It  will 
be  found  that  the  atomic  theory  of  the  constitution  of  mat- 
ter exhibits  the  mechanism  of  the  process  by  which  that 
capacity  of  the  great  luminary  for  supplying  the  radiation 
so  vital  to  the  welfare  of  mankind  is  sustained  from  age 
to  age. 

Let  me  here  anticipate  an  objection  which  may  not  im- 
probably be  raised.    Those  who  have  paid  attention  to  this 
subject  are  aware  that  the  remarkable  doctrine  first  pro- 
pounded by  Helmholtz  removed  all  real  doubt  from  the 
matter.     It  is  to  this  eminent  philosopher  we  owe  an  ex- 
planation of  what  at  first  seemed  to  be  a  paradox.    He  ex- 
1  From  the  "  Fortnightly  Review,"  October,  1893. 
3 


4  BALL 

plained  how,  notwithstanding  that  the  sun  radiates  its  heat 
so  profusely,  no  indications  of  the  inevitable  decline  of  heat 
can  be  as  yet  discovered.  If  the  sun  had  been  made  of 
solid  coal  from  centre  to  surface,  and  if  that  coal  had  been 
burned  for  the  purpose  of  sustaining  the  radiation,  it  can 
be  demonstrated  that  a  few  thousand  years  of  solar  expendi- 
ture at  the  present  rate  would  suffice  to  exhaust  all  the 
heat  which  the  combustion  of  that  great  sphere  of  fuel 
could  generate.  We  know,  however,  that  the  sun  has  been 
radiating  heat,  not  alone  for  thousands  of  years,  but  for  mil- 
lions of  years.  The  existence  of  fossil  plants  and  animals 
would  alone  suffice  to  demonstrate  this  fact.  We  have  thus 
to  account  for  the  extremely  remarkable  circumstance  that 
our  great  luminary  has  radiated  forth  already  a  thousand 
times  as  much  heat  as  could  be  generated  by  the  combus- 
tion of  a  sphere  of  coal  as  big  as  the  sun  is  at  present,  and 
yet,  notwithstanding  this  expenditure  in  the  past,  physics 
declares  that  for  millions  of  years  to  come  the  sun  may  con- 
tinue to  dispense  light  and  heat  to  its  attendant  worlds 
with  the  same  abundant  prodigality.  To  have  shown  how 
the  apparent  paradox  could  be  removed  is  one  of  the  most 
notable  achievements  of  the  great  German  philosopher. 

What  Helmholtz  did  was  to  refer  to  the  obvious  fact 
that  the  expenditure  of  heat  by  radiation  must  necessarily 
lead  to  shrinkage  of  the  solar  volume.  This  shrinkage  has 
the  effect  of  abstracting  from  a  store  of  potential  energy 
in  the  sun  and  transforming  what  it  takes  into  the  active 
form  of  heat.  The  transformation  advances  pari  passu  with 
the  radiation,  so  that  the  loss  of  heat  arising  from  the  radia- 
tion is  restored  by  the  newly  produced  heat  derived  from 
the  latent  reservoir.  Such  is  an  outline  of  the  now  famous 
doctrine  universally  accepted  among  physicists.  It  fulfils 
the  conditions  of  the  problem,  and  when  tested  by  arith- 
metical calculation  it  is  not  found  wanting. 

But  the  genuine  student  of  Nature  loves  to  get  to  the 
heart  of  a  great  problem  like  this;  he  loves  to  be  able  to 
follow  it,  not  through  mere  formulae  or  abstract  principles, 
but  so  as  to  be  able  to  visualize  its  truth  and  feel  its  cer- 


ATOMS   AND   SUNBEAMS  5 

tainty.  He  will,  therefore,  often  desire  something  in  addi- 
tion to  the  bare  presentation  of  the  theory  as  above  stated. 
It  may  be  no  doubt  sufficient  for  the  mathematician  to 
know  that  the  total  potential  energy  in  the  sun,  due  to  the 
dispersed  nature  of  its  materials,  is  so  vast  that  as  contrac- 
tion brings  the  materials,  on  the  whole,  somewhat  nearer 
together,  the  potential  energy  thus  surrendered  is  trans- 
formed into  a  supply  of  heat  quite  adequate  to  compensate 
for  the  losses  arising  from  the  radiation  by  which  the  con- 
traction was  produced.  The  student  who  admits — and  who 
is  there  that  does  not  admit? — the  doctrine  of  the  conserva- 
tion of  energy  knows  that  in  this  argument  he  is  on  thor- 
oughly reliable  ground.  At  the  same  time  the  argument 
does  not  actually  offer  any  very  clear  conception,  or  indeed 
any  conception  at  all,  of  the  precise  modus  operandi  by 
which,  as  the  active  potential  energy  vanishes,  its  equivalent 
in  available  heat  appears.  I  have  always  felt  that  this  was 
the  unsatisfactory  part  of  an  otherwise  perfect  theory.  It 
was,  therefore,  with  much  interest  that  I  became  acquainted 
a  short  time  ago  with  a  development  of  the  molecular 
theory  of  gases  which  afforded  precisely  what  seemed 
wanted  to  make  every  link  in  the  chain  of  the  great  argu- 
ment distinctly  perceptible.  I  make  no  doubt  that  the  no- 
tions which  have  occurred  to  me  on  this  subject  must  have 
presented  themselves  to  others  also.  I  have,  however,  not 
read  in  print  or  heard  in  conversation  any  use  made  of  the 
illustration  that  I  am  going  to  set  forth.  I  feel,  therefore, 
confident  that  even  if  it  be  known  at  all,  it  is  certainly  not 
generally  known  among  the  large  and  ever-increasing  circle 
of  readers  to  whom  the  great  questions  of  physics  are  of 
interest. 

The  division  of  matter  into  the  three  forms  of  solids, 
liquids,  and  gases  has  acquired  in  these  days  a  special  sig- 
nificance now  that  the  constitution  of  matter  is  becoming 
in  some  degree  understood.  First  let  it  be  noted  that, 
though  matter  is  capable  of  subdivision  to  a  certain  extent, 
yet  that  there  is  a  limit  beyond  which  subdivision  could  not 
be  carried.  This  statement  touches  upon  the  ancient  con- 


6  BALL 

troversy  as  to  the  infinite  divisibility  of  matter.  Even  still 
we  can  find  the  statement  in  some  of  our  old  text-books 
that  there  is  no  particle  of  matter  so  small  that  it  could  not 
be  again  subdivided  into  half.  No  doubt,  so  far  as  most 
ordinary  experience  goes,  this  statement  may  be  unques- 
tionable. It  is  quite  true  that  we  do  not  often  reduce  mat- 
ter to  fragments  so  small  that  each  of  them  shall  be  insus- 
ceptible of  further  conceivable  division.  But,  to  illustrate 
the  natural  principle  now  under  consideration,  let  us  take 
the  example  of  a  body  which  is  itself  composed  of  but  a 
single  element.  Think,  for  instance,  of  a  diamond,  which 
is,  as  we  all  know,  a  portion  of  crystallized  carbon.  It  is 
true  that  the  reduction  of  diamonds  to  powder  is  a  labori- 
ous process.  Still,  diamond  dust  has  to  be  produced  in  the 
finishing  of  the  rough  stone,  and  this  element  will  serve  the 
purpose  of  our  present  argument  better  than  a  substance 
of  a  composite  nature.  Each  particle  of  the  diamond  dust 
is,  of  course,  as  much  a  particle  of  carbon  as  was  the 
original  crystal.  We  may,  however,  suppose  that  by  a  repe- 
tition of  the  process  a  reduction  of  the  diamond  dust  to 
powder  still  finer  is  accomplished.  The  grains  thus  ob- 
tained may  have  become  so  minute  that  they  have  ceased 
to  be  visible  to  the  unaided  eye,  and  require  a  microscope 
to  render  them  perceptible;  but  even  after  this  comminu- 
tion each  of  these  particles  is  still  a  veritable  diamond.  It 
possesses  the  properties,  optical,  chemical,  and  mechanical, 
of  the  original  gem,  from  which  it  differs  merely  in  the 
attribute  of  size.  Even  when  the  disintegration  has  been 
carried  to  such  a  point  that  each  individual  particle  can  be 
only  just  perceived  by  the  keenest  power  of  the  most  pow- 
erful microscope,  there  is  still  no  indication  that  the  par- 
ticles cease  to  possess  the  characteristics  of  the  original 
body.  These  facts  being  undoubted,  it  was  perhaps  not 
unnatural  to  suppose  that  the  reduction  could  be  carried  on 
indefinitely,  and  that  even  if  the  smallest  fragment  of  dia- 
mond which  could  be  seen  in  a  powerful  microscope  were 
reduced  to  a  millionth  part,  and  each  of  those  to  a  million 
more,  yet  that  the  ultimate  particles  thus  reached  would  be 


ATOMS   AND   SUNBEAMS  7 

diamonds  still.  Now,  however,  we  know  that  that  is  not 
the  case.  The  smallest  particle  visible  under  a  microscope 
might  indeed  be  crushed  into  a  thousand  parts,  and  each 
one  of  those  parts,  though  wholly  inappreciable  to  our 
sense  of  touch  or  vision,  would  nevertheless  be  a  genuine 
diamond.  If,  however,  the  subdivision  be  carried  on  until 
the  particles  produced  are,  roughly  speaking,  one-millionth 
part  of  the  bulk  of  the  smallest  objects  which  could  be  seen 
in  the  microscope,  we  then  approach  the  limits  of  partition 
of  which  the  diamond  would  be  susceptible.  We  now 
know  that  there  is  an  atom  of  diamond  so  small  that  it  must 
refuse  to  undergo  any  further  division.  This  ultimate  atom, 
be  it  observed,  is  not  an  infinitely  small  quantity.  It  has 
definite  dimensions;  it  possesses  a  definite  weight.  All  such 
diamond  atoms  are  precisely  alike  in  weight,  and  probably  in 
other  characteristics.  It  might  be  thought  that  if  this  atom 
has  finite  dimensions,  it  is,  at  all  events,  conceivable  that  it 
should  admit  of  further  subdivision.  In  a  certain  sense  this 
is,  no  doubt,  the  case.  The  diamond  atom  is  made  up  of 
parts  and,  being  so  made,  it  is,  of  course,  conceivable  that 
those  parts  could  be  separated.  The  important  point  to 
notice  is,  that  no  means  known  to  us  could  produce  this 
separation,  while  it  is  perfectly  certain  that  if  the  decom- 
position of  the  atom  of  diamond  into  distinct  parts  could 
be  effected,  those  parts  would  not  be  diamonds  at  all,  nor 
anything  in  the  least  resembling  diamonds. 

What  we  have  said  as  regards  the  element  carbon  may 
be  extended  to  every  other  elementary  substance.  Sulphur 
is  familiarly  known  in  a  form  of  extreme  subdivision,  and 
each  little  particle  of  sulphur  could  be  further  comminuted 
to  a  certain  point  beyond  which  any  further  partition  would 
be  impossible.  So,  too,  any  composite  body — such,  for  ex- 
ample, as  a  lump  of  sugar — admits  of  being  decomposed 
into  molecules  so  small  that  any  further  separation  would 
be  impossible  if  the  molecule  were  still  to  remain  sugar. 
No  doubt,  a  separation  of  the  molecule  of  any  composite 
body  into  constituent  atoms  of  other  elements  is  not  alone 
possible,  but  is  incessantly  taking  place. 


8  BALL 

The  first  step  in  our  knowledge  of  the  constitution  of 
matter  has  been  taken  when  we  have  come  to  recognise 
that  every  body  is  composed  of  a  multitude  of  extremely, 
but  not  infinitely,  small  molecules.  The  next  point  relates 
to  the  condition  in  which  these  molecules  are  found.  At 
first  it  might  be  thought  that  in  a  solid,  at  all  events,  the 
little  particles  must  be  clustered  together  in  a  compact 
mass.  If  we  depended  merely  on  sensible  evidence  it  would 
seem  that  a  lump  of  iron,  if  constituted  from  molecules  at 
all,  must  be  simply  a  cohering  mass  of  particles,  just  as  a 
multitude  of  particles  of  sand  unite  to  form  a  lump  of  sand- 
stone. But  the  truth  is  far  more  wonderful  than  such  a 
belief  would  imply.  Were  the  sensibility  of  our  eyes  so 
greatly  increased  as  to  make  them  a  few  million  times  more 
powerful  than  our  present  organs,  then,  indeed,  the  display 
of  the  texture  of  solid  matter  would  be  an  astonishing  reve- 
lation. It  would  be  seen  that  the  diamond  atoms,  which, 
when  aggregated  in  sufficient  myriads,  form  the  perfect 
gem,  were  each  in  a  condition  of  rapid  movement  of  the 
most  complex  description;  each  molecule  would  be  seen 
swinging  to  and  fro  with  the  utmost  violence  among  the 
neighbouring  molecules.  It  would  be  seen  quivering  all 
over  under  the  influence  of  the  shocks  which  it  would  re- 
ceive from  the  vehement  encounters  with  other  molecules 
which  occur  millions  of  times  in  each  second.  Such  would 
be  the  minute  anatomy  of  the  diamond.  The  well-known 
properties  of  such  gems  seem,  at  first  sight,  wholly  at 
variance  with  the  curious  structure  we  have  assigned  to 
them.  Surely,  it  may  be  said  that  the  hardness  and  the 
impenetrability  so  characteristic  of  the  diamond  refute  at 
once  the  supposition  that  it  is  no  more  than  a  cluster  of 
rapidly  moving  particles.  But  the  natural  philosopher  now 
knows  that  his  explanation  of  the  qualities  of  the  diamond 
holds  the  field  against  all  other  explanations.  The  well- 
known  impenetrability  of  the  diamond  seems  to  arise  from 
the  fact  that  when  you  try  to  press  a  steel  point  into  the 
stone  you  fail  to  do  so  because  the  rapidly  moving  mole- 
cules of  the  gem  batter  the  end  of  the  steel  point  with  such 


ATOMS   AND  SUNBEAMS  9 

extraordinary  vehemence  that  they  refuse  to  allow  it  to 
penetrate  or  even  to  mark  the  crystallized  surface.  When 
you  cut  glass  with  a  diamond  it  is  quite  true  that  the  edge, 
which  seems  so  intensely  hard,  is  really  composed  of  rapidly 
moving  atoms.  But  the  glass  which  is  submitted  to  the 
operation  is  also  merely  a  mass  of  moving  molecules,  and 
what  seems  to  happen  is,  that,  as  the  diamond  is  pressed 
forward,  its  several  particles,  by  their  superior  vigour,  drive 
the  little  particles  of  glass  out  of  the  way.  We  do  not  see 
the  actual  details  of  the  myriad  encounters  in  which  the 
diamond  atoms  are  victorious  over  the  glassy  molecules; 
we  only  discern  the  broad  result  that  the  diamond  has  done 
its  work,  and  that  the  glass  has  been  cut. 

It  may  well  be  asked  how  we  know  that  matter  is  con- 
stituted of  molecules  in  intensely  rapid  movement.  The 
statement  seems  at  the  first  glance  to  be  so  utterly  at  vari- 
ance with  our  ordinary  experience  that  we  demand,  and 
rightly  demand,  some  convincing  proof  on  the  matter. 
There  are  many  arguments  by  which  the  required  demon- 
stration can  be  forthcoming.  The  one  which  I  shall  give 
is  not  perhaps  the  most  conclusive,  but  it  has  the  advantage 
of  being  one  of  the  simplest  and  the  most  readily  intel- 
ligible. 

Let  us  see  if  we  can  not  prove  at  once  that  the  mole- 
cules in,  let  us  say,  a  piece  of  iron  must  be  in  movement. 
Suppose  that  the  iron  is  warmed  so  that  it  radiates  heat  to 
a  perceptible  extent.  We  know  that  the  heat  which,  in 
this  case,  affects  our  nerves  has  been  transmitted  from  its 
origin  by  ethereal  undulations.  Those  undulations  have, 
undoubtedly,  been  set  in  motion  by  the  iron,  and  yet  the 
parts  of  the  metal  seem  quite  motionless  relatively  to  each 
other,  notwithstanding  that  they  possess  the  power  of  set- 
ting the  ether  into  vibration.  It  is  impossible  that  such 
vibrations  could  be  produced  were  it  not  that  there  is  in 
the  iron  a  something  which  vibrates  in  such  a  manner  as  to 
communicate  the  necessary  pulses  to  the  ether.  It  there- 
fore follows  that  in  the  texture  of  the  solid  iron  there  must 
be  some  molecular  movement,  timed  in  such  a  way  as  to 


10  BALL 

impart  to  the  ether  the  actual  vibrations  which  we  find  it 
to  possess.  The  argument  in  this  case  may  be  illustrated 
by  the  analogous  phenomena  presented  in  the  case  of 
sound.  As  we  listen  to  the  notes  of  a  violin,  what  we 
actually  perceive  are  vibrations  communicated  through  the 
air  to  the  auditory  apparatus.  We  can  trace  these  aerial 
vibrations  back  to  their  source,  and  we  find  they  originate 
from  the  quivering  of  the  violin  under  the  influence  of  the 
bow  of  the  performer.  Were  it  not  for  these  vibrations  of  the 
instrument  the  aerial  vibrations  would  not  be  produced,  and 
the  corresponding  sounds  would  not  be  heard.  Far  more 
delicate  than  the  atmospheric  waves  of  sound  are  the  ethe- 
real waves  corresponding  to  light  or  to  heat,  but  none  the 
less  must  these  latter  also  originate  from  the  impulse  of 
some  vibrating  mass.  It  is  thus  apparent  that  a  hot  piece 
of  iron,  however  still  it  may  seem,  must  be  animated  by  an 
excessively  rapid  molecular  movement.  Nor  is  the  validity 
of  this  conclusion  impaired  even  if  the  iron  be  at  ordinary 
temperature.  We  know  that  a  body  which  is  no  hotter 
than  the  surrounding  bodies  is  still  incessantly  radiating 
heat  to  them  and  receiving  heat  from  them  in  return.  Thus 
we  are  led  to  the  conviction  that  a  piece  of  iron,  whatever 
be  its  temperature,  must  consist  of  atoms  in  a  state  of  lively 
movement.  The  important  conclusion  thus  drawn  with 
regard  to  iron  may  be  equally  stated  with  respect  to  every 
other  solid,  or,  indeed,  every  other  body,  whether  solid, 
liquid,  or  gaseous.  All  matter  of  every  description  is  not 
only  known  to  be  composed  of  molecules,  but  it  is  also  now 
certain  that  those  molecules  are  incessantly  performing 
movements  of  a  very  complex  type. 

A  closer  study  of  this  subject  will  be  necessary  for  our 
present  purpose,  and  it  will  be  convenient  to  examine 
matter  in  that  state  in  which  it  is  exhibited  in  its  very  sim- 
plest type  from  the  molecular  point  of  view.  This  condi- 
tion is  not  presented,  as  might  at  first  be  supposed,  when 
the  matter  is  solid,  like  a  diamond,  or  like  a  piece  of  iron. 
Even  in  a  liquid  the  complexity  of  molecular  constitution, 
though  somewhat  less  than  in  the  case  of  a  solid,  is  still 


ATOMS  AND  SUNBEAMS  II 

notably  greater  than  in  matter  which  has  the  gaseous  form. 
The  air  that  we  breathe  is  matter  almost  of  the  most  simple 
kind,  so  far  as  ^molecular  constitution  is  concerned.  It 
should,  however,  be  noted  that,  as  air  consists  of  a  mixture, 
it  would  be  better  for  our  purpose  to  think  of  a  gas  isolated 
from  any  other  element.  Let  us  take  the  case  of  oxygen, 
the  most  important  constituent  of  our  atmosphere. 

Like  every  other  element,  oxygen  is  composed  of  mole- 
cules, and  those  molecules  are  in  a  state  of  rapid  motion. 
It  might  be  expected  that  the  affinity  by  which  the  different 
molecules  were  allied  in  the  case  of  a  gas  should  be  of  the 
simplest  nature,  and  this  is  indeed  found  to  be  the  case. 
Notwithstanding  that  oxygen  is  an  invisible  body,  and  not- 
withstanding that  the  molecules  are  so  excessively  minute 
as  to  be  severally  quite  inappreciable  to  our  senses,  yet  we 
have  been  able  to  learn  a  great  deal  with  regard  to  the  con- 
stitution of  the  molecules  of  this  gas.  The  mental  eye  of 
the  philosopher  shows  him  that,  though  the  oxygen  with 
which  a  jar  is  filled  appears  to  be  perfectly  quiescent,  yet 
that  quiescence  has  there  no  real  existence.  He  knows  that 
oxygen  consists  of  myriads  of  molecules  identical  in  weight 
and  in  other  features,  and  darting  about  one  among  the 
other  with  velocities  which  vary  perhaps  between  those  of 
express  trains  and  those  of  rifle  bullets.  He  sees  that  each 
little  molecule  hurries  along  quite  freely  for  a  while  until 
it  happens  to  encounter  some  other  molecule  equally  bent 
on  its  journey,  and  then  a  collision  takes  place.  Perhaps  it 
would  be  more  correct  to  say  that  what  usually  happens  is 
that  the  two  impinging  molecules  make  a  very  close  ap- 
proach; then  each  of  them  so  vehemently  attracts  the  other 
as  to  make  it  swerve  out  of  its  course  and  start  it  off  along 
a  path,  inclined,  it  may  be,  even  at  a  right  angle  to  that 
which  it  previously  pursued.  The  molecules  in  a  gas  at 
ordinary  pressures  are  so  contiguous  that  these  encounters 
take  place  incessantly;  in  fact,  we  are  able  to  show  that 
each  individual  molecule  will  probably  experience  such  ad- 
ventures some  millions  of  times  in  the  course  of  each  sec- 
ond. We  are  able  to  calculate  the  average  velocity  with 


12  BALL 

which  the  several  molecules  move  when  the  gas  has  a 
certain  temperature.  We  know  how  to  determine  the 
average  length  of  the  free  path  which  each  molecule  trav- 
erses in  the  interval  between  two  consecutive  encounters. 
We  are  able  to  trace  how  all  these  circumstances  would 
vary  if,  instead  of  oxygen  gas,  we  took  nitrogen,  or  hydro- 
gen, or  any  other  body  in  the  same  molecular  state.  It  is, 
in  fact,  characteristic  of  every  gas  that  each  molecule  wan- 
ders freely,  subject  only  to  those  incessant  encounters  with 
other  similar  wanderers  by  which  its  path  is  so  frequently 
disturbed.  If  two  gases  be  placed  in  the  same  vessel,  one 
being  laid  over  the  other,  it  will  presently  be  found  that  the 
two  gases  begin  to  blend;  ere  long  one  gas  will  have  dif- 
fused uniformly  through  the  latter,  so  that  the  two  will  have 
become  a  perfect  mixture  just  as  the  oxygen  and  nitrogen 
have  done  in  our  own  atmosphere.  The  molecular  theory 
of  gases  explains  at  once  the  actual  character  of  the  opera- 
tion by  which  diffusion  is  effected.  Across  the  boundary 
which  initially  separates  the  two  gases  certain  molecules  are 
projected  from  either  side,  and  this  process  of  interchange 
goes  on  until  the  molecules  become  uniformly  distributed 
throughout. 

There  is,  indeed,  nothing  more  remarkable  than  the  fact 
that  information  so  copious  and  so  recondite  can  be  ob- 
tained in  a  region  which  lies  altogether  beyond  the  direct 
testimony  of  the  senses.  Just  as  the  astronomer  staggers 
our  powers  of  conception  by  the  description  of  appalling 
distances  and  stupendous  periods  of  time,  and  relies  with 
confidence  on  the  evidence  which  convinces  him  of  the 
reality  of  his  statements,  so  the  physicist  avails  himself  of 
a  like  potent  method  of  research  to  study  distances  so 
minute  and  times  so  brief  that  the  imagination  utterly  fails 
to  realize  them. 

In  the  case  of  a  liquid,  the  freedom  enjoyed  by  the  mole- 
cules is  considerably  more  restricted  than  in  the  case  of  a 
gas.  It  would  seem  that  in  the  denser  fluid  there  can  be 
no  intervals  of  undisturbed  travel  permitted  to  a  molecule; 
it  is  almost  incessantly  in  a  state  of  encounter  with  some 


ATOMS  AND  SUNBEAMS  13 

other  similar  object.  When  a  molecule  in  a  liquid  breaks 
away  from  its  association  with  one  group,  it  is  only  because 
it  has  entered  into  alliance  with  another.  As,  however,  two 
liquids  will  very  frequently  blend  if  so  placed  that  diffusion 
be  possible,  we  have  a  proof  that,  though  the  transference 
of  a  particular  molecule  through  the  liquid  may  be  com- 
paratively slow,  yet  it  will  gradually  exchange  association 
with  one  group  for  association  with  another,  and  may  in 
this  way  travel  throughout  any  distance  to  which  the  liquid 
extends. 

In  the  case  of  a  solid  there  is  still  further  limitation  im- 
posed on  the  mobility  of  each  separate  molecule.  It  is  now 
no  longer  permitted  to  make  excursions  throughout  the 
entire  volume  of  the  body.  Each  molecule  is  in  rapid 
motion,  it  is  true,  but  those  movements  are  confined  to 
gyrations  within  minutely  circumscribed  limits.  Two  solids 
placed  in  contact  do  not  generally  diffuse  one  into  the 
other,  the  incapacity  for  diffusion  being  the  direct  conse- 
quence of  the  inferior  degree  of  mobility  possessed  by  the 
molecules  in  this  condition  of  matter. 

It  is  known  that  the  immediate  effect  of  the  application 
of  heat  is  to  increase  the  velocities  with  which  the  molecules 
move.  Apply  heat,  for  instance,  to  the  water  in  a  kettle; 
the  moving  molecules  of  water  are  thereby  stimulated  to 
even  greater  activity,  and  it  will  occasionally  happen  that 
the  velocity  thus  acquired  by  a  molecule  becomes  so 
great  that  the  little  particle  will  swing  clear  away  from  the 
influence  of  the  other  molecules  with  which  it  had  been 
associated.  When  this  takes  place  in  the  case  of  a  suf- 
ficient number  of  molecules,  they  dart  freely  from  the  sur- 
face of  the  liquid,  thus  producing  the  effect  which  in  our 
ordinary  language  we  describe  as  giving  off  steam.  If, 
therefore,  a  volume  of  gas  be  heated,  the  velocities  with 
which  its  molecules  are  animated  will  be  in  general  in- 
creased. As  the  molecular  velocities  throughout  the  ex- 
tent of  the  gas  are,  on  the  whole,  augmented,  it  is  quite 
plain  that  the  intensities  of  the  shocks  experienced  by  the 
molecules  in  their  several  encounters  will  be  also  accentu- 


I4  BALL 

ated.  The  more  rapidly  moving  particles  will  strike  each 
against  the  other  with  increased  violence,  and  the  contem- 
plation of  this  single  fact  leads  us  close  to  one  of  Nature's 
greatest  secrets. 

Let  us  think  of  the  abounding  heat  which  is  dispensed 
to  us  from  the  sun.  That  heat  comes,  as  we  know,  in  the 
form  of  undulations  imparted  to  the  ether  by  the  heated 
matter  in  the  sun,  and  transmitted  thence  across  space  for 
the  benefit  of  the  earth  and  its  inhabitants.  I  have  already 
explained  that  these  vibrations  in  the  ether  must  take  their 
rise  from  molecular  movements,  and  it  is  important  to  no- 
tice that  the  character  of  the  vibrations  in  the  ether  enable 
us  to  learn  to  some  extent  the  precise  description  of  mo- 
lecular movements  which  alone  would  be  competent  to 
produce  the  particular  vibrations  corresponding  to  radiant 
heat.  At  first  it  might  be  thought  that  it  was  the  rapid 
movements  of  translation  of  the  molecules  themselves,  as 
entire  if  extremely  minute  bodies,  which  caused  the  ethe- 
real vibration,  but  this  is  not  so.  We  must  carefully  ob- 
serve that  there  is  another  kind  of  molecular  motion  be- 
sides that  which  the  molecule  possesses  as  a  whole.  We 
have  hitherto  been  occupied  only  with  the  movements  of 
each  molecule  as  a  little  projectile  pursuing  its  zigzag 
course,  each  turn  of  the  zigzag  being  the  result  of  an  en- 
counter with  some  similar  molecule  belonging  to  the  same 
medium.  But  we  have  now  to  observe  that  the  molecule 
itself  is  by  no  means  to  be  regarded  as  a  simple  rigid  par- 
ticle; indeed,  if  it  were  so,  it  is  certain  that  we  should  re- 
ceive no  heat  at  all  from  the  sun.  We  have  the  best  reasons 
for  believing  that  the  molecule  of  matter,  so  far  from  re- 
sembling a  simple  rigid  particle,  is  an  elaborate  structure, 
whose  parts  are  in  some  degree  capable  of  independent 
movement.  It  will  not,  indeed,  be  necessary  for  us  to  adopt 
the  splendid  hypothesis  of  Lord  Kelvin,  which  supposes 
that  molecules  of  matter  are  merely  vortex  rings  in  that 
perfect  fluid,  the  ether.  It  seems  difficult  to  doubt  that  this 
doctrine  represents  the  facts,  but  if  any  one  should  reject  it, 
then  I  have  only  to  say  that  its  assumption  is  not  required 


ATOMS  AND  SUNBEAMS  15 

for  our  present  argument.  All  that  is  necessary  for  us  is 
to  regard  each  molecule  as  somewhat  resembling  an  elastic 
structure  made  of  parts  which  can  quiver  like  springs,  and 
so  arranged  as  to  be  susceptible  of  many  different  modes 
of  vibration.  We  are  to  suppose  that  each  molecule,  in 
addition  to  the  energy  which  it  possesses  in  virtue  of  its 
movement  of  translation  as  a  whole,  has  also  a  store  of 
energy  corresponding  to  the  oscillations  of  its  electric 
springs.  We  can,  in  fact,  in  some  cases  determine  the  ratio 
which  exists  between  the  amount  of  energy  which  is,  on 
the  average,  possessed  by  molecules  in  consequence  of  their 
velocities  of  translation,  and  the  amount  of  energy  which 
they  possess  in  consequence  of  the  vibrations  by  which  their 
several  parts  are  animated.  It  is  these  internal  molecular 
vibrations  which  are  of  essential  importance  in  our  present 
inquiry.  It  is  believed  that  the  radiation  of  light,  or  of  heat, 
generally  takes  rise  in  the  impulses  given  to  ether  by  the 
internal  molecular  vibrations.  Do  we  not  know  that  the 
essential  characteristic  of  those  ethereal  movements  which 
correspond  to  radiant  light  and  heat  is  that  they  have  the 
nature  of  oscillations?  Such  could  not  be  imparted  by 
mere  rectilinear  movements  of  the  molecules  as  a  whole. 
They  must  be  due  to  those  internal  oscillations  by  which 
the  actual  molecules  are  animated. 

No  doubt  it  is  difficult  to  realize  that  much  can  be 
learned  with  regard  to  the  performances  that  actually  go 
on  in  the  internal  parts  of  a  molecule,  especially  when  it  is 
remembered  that  each  molecule  in  its  entirety  is  so  ex- 
tremely minute  as  to  be  entirely  beyond  the  reach  of  our 
organs  of  sense.  It  is,  nevertheless,  impossible  to  doubt 
that  the  statements  just  made  correspond  to  the  veritable 
facts  of  Nature.  It  would  be  impracticable  here  to  go  into 
any  complete  detail  with  regard  to  the  evidence  on  this  sub- 
ject; I  can  only  sketch  an  outline  of  it.  Let  us  take,  per- 
haps as  the  simplest  case,  that  presented  by  hydrogen. 

At  the  ordinary  temperature  of  the  air  hydrogen  is,  of 
course,  invisible;  this  means  that  the  vibrations  in  the  in- 
terior of  the  molecules  are  not  sufficiently  vehement  to  im- 


1 6  BALL 

" 

part  pulses  to  the  ether  with  the  energy  that  would  be  re- 
^__  quired  to  produce  visual  effects.  Now,  let  us  suppose  that 
the  hydrogen  is  heated.  The  effect  of  heating  is  to  impart 
additional  speed  to  the  molecules  of  the  gas,  and  conse- 
quently when  the  molecules  happen  to  come  together  their 
encounter  is  more  violent.  The  effect  of  such  an  occur- 
rence on  one  of  these  little  elastic  bodies  is  to  set  it  quiver- 
ing with  greater  vehemence  in  those  particular  modes  of 
vibration  for  which  it  is  tuned.  If  the  temperature  of  the 
gas  has  been  raised  sufficiently  high,  as  it  can  be  by  the  aid 
of  electricity,  then  the  internal  energy  acquired  by  the  mole- 
cules, in  consequence  of  the  increased  vehemence  of  their 
collisions,  has  become  so  great  that  they  are  able  to  impart 
pulses  to  the  ether  with  sufficient  intensity  to  affect  our 
nerves  of  vision;  thereupon  we  declare  that  the  hydrogen 
is  now  so  hot  as  to  have  become  luminous.  Suppose  we 
employ  a  spectroscope  for  the  purpose  of  studying  the  par- 
ticular character  of  the  light  which  the  glowing  hydrogen 
dispenses.  It  will  appear  that  the  spectrum  consists  of  a 
definite  number  of  bright  lines.  We  know  that  each  one  of 
these  lines  corresponds  to  a  particular  period  of  vibration 
of  the  ether,  and  hence  we  see  that  the  light  emitted  by  the 
hydrogen  does  not  consist  of  vibrations  of  all  periods  indis- 
criminately, but  only  of  certain  particular  waves  which  are 
in  unison  with  the  oscillations  to  which  the  internal  parts  of 
the  molecule  of  hydrogen  are  adapted.  Had  we  examined 
the  spectrum  of  some  other  gas  in  a  state  of  incandescence 
we  should  have  found  a  wholly  different  system  of  lines 
from  those  pertaining  to  hydrogen.  This  demonstrates 
that  the  molecules  of  one  gas  differ  essentially  from  those 
of  another  in  respect  to  the  character  of  the  internal  vibra- 
tions which  they  are  adapted  to  perform.  The  extraordi- 
nary activity  of  the  movements  which  take  place  within  the 
molecules  may  be  appreciated  from  the  following  facts: 
We  know  that  the  wave  corresponding  to  one  of  the  hydro- 
gen lines  has  a  length  of  about  the  forty-thousandth  of  an 
inch;  we  also  know  that  in  a  single  second  of  time  light 
travels  over  a  space  of  a  hundred  and  eighty-six  thousand 


ATOMS  AND  SUNBEAMS  17 

miles;  a  simple  calculation  will,  therefore,  assure  us  that 
certain  vibrations  in  the  molecules  of  hydrogen  correspond- 
ing to  this  particular  undulation  must  take  place  with  such 
an  extraordinary  frequency  that  about  four  hundred  and 
sixty  millions  of  millions  of  them  are  performed  in  each 
second  of  time. 

Provided  with  these  conceptions,  we  shall  now,  I  think, 
be  able  to  see  without  difficulty  how  it  is  that  the  sun's  heat 
is  sustained.  We  may,  for  our  present  purpose,  think  of  the 
great  luminary  as  a  mass  of  glowing  gas.  It  is  quite  true 
that  the  physical  condition  of  the  matter  in  the  interior  of 
the  tremendous  globe  can  hardly  be  that  which  we  ordi- 
narily consider  as  gaseous.  But  this  need  not  affect  our 
argument.  It  is  undoubtedly  true  that  those  portions  of 
the  solar  atmosphere  from  which  the  light  and  heat  are 
mainly  dispensed  are  gaseous  in  their  character,  or,  at  all 
events,  come  sufficiently  near  to  matter  in  the  gaseous  state 
to  permit  the  application  of  the  line  of  argument  with  which 
we  have  hitherto  been  engaged.  In  consequence  of  the 
vast  mass  of  the  sun  the  gravitation  with  which  it  draws  all 
bodies  toward  it  is  very  much  greater  than  the  gravitation 
on  the  surface  of  the  earth.  On  our  globe  we  know  that 
the  effect  of  gravitation  is  to  impart  to  any  body  near  the 
surface  velocity  directed  toward  the  earth's  centre  at  the 
rate  of  thirty-two  feet  per  second.  The  sun  is  more  than 
three  hundred  thousand  times  as  massive  as  the  earth;  we 
can  not,  however,  assert  that  the  gravitation  is  increased  in 
the  same  proportion,  because,  on  account  of  the  vast  size 
of  the  sun,  a  particle  at  its  surface  is  more  than  a  hundred 
times  farther  away  from  the  solar  centre  than  a  body  on 
the  surface  of  the  earth  is  from  the  terrestrial  centre.  It 
can,  however,  be  shown  that,  taking  these  various  matters 
into  account,  the  actual  intensity  of  gravitation  at  the  solar 
surface  is  sufficient  to  tend  to  impart  to  all  objects  an  in- 
crease of  velocity  toward  the  sun's  centre  at  the  rate  of  four 
hundred  and  fifty-seven  feet  per  second.  This  would  apply 
not  only  to  a  meteorite,  or  other  considerable  mass,  which 
is  falling  into  the  sun;  it  would  be  equally  true  of  an  object 


!8  BALL 

as  small  as  a  molecule.  Every  one  of  the  myriads  of  gase- 
ous molecules  in  the  outer  regions  of  the  solar  atmosphere 
must  be  constantly  acted  upon  by  this  attractive  force, 
which  tends  in  the  course  of  each  second  to  add  to  them  a 
downward  velocity  at  that  rate  per  second  which  has  already 
been  stated.  It  is  quite  true  that  to  a  great  extent  the  effect 
of  this  attraction  is  masked  by  counteracting  tendencies. 
In  particular  we  may  mention  that,  inasmuch  as  the  den- 
sity of  the  solar  atmosphere  increases  as  the  sun's  centre 
is  approached,  the  flying  molecule  generally  finds  itself 
more  obstructed  by  encounters  with  other  molecules  when 
it  is  descending  than  when  it  is  ascending.  We  may  here 
contrast  the  condition  of  the  atmosphere  on  the  earth  with 
the  condition  of  the  solar  atmosphere.  Each  molecule  in 
our  air,  being  acted  upon  by  terrestrial  gravitation,  has 
thereby  a  tendency  to  fall  downward  with  a  velocity  con- 
tinually increasing  at  the  rate  of  thirty-two  feet  per  second. 
As,  however,  the  terrestrial  atmosphere  has  long  since 
reached  a  stable  condition,  in  which  it  undergoes  no  further 
contraction,  the  effect  of  gravitation  in  adding  velocity  to 
the  molecules  is  so  completely  masked  by  the  counteracting 
tendencies  that,  on  the  whole,  there  is  no  continual  increase 
of  molecular  velocities  downward  due  to  gravitation.  Were 
such  an  increase  at  present  going  on,  we  should  necessarily 
find  that  the  terrestrial  atmosphere  was  decreasing  in  vol- 
ume, and  ever  becoming  more  condensed  in  its  lower  strata. 
It  is,  however,  well  known  that  no  such  changes  as  are  here 
implied  are  taking  place.  The  essential  difference  between 
the  earth  and  the  sun,  so  far  as  the  matter  now  before  us  is 
concerned,  is  to  be  found  in  the  fact  that,  as  the  sun  has  not 
yet  passed  into  the  form  of  a  rigid  body,  it  is  still  contract- 
ing at  a  rate  very  much  greater  than  that  at  which  a  body 
grown  so  cold  as  the  earth  draws  its  particles  closer  to- 
gether. The  molecules  in  the  solar  photosphere  accord- 
ingly yield  to  a  certain  extent  to  the  gravitation  which 
constantly  seeks  to  draw  them  down.  The  counteracting 
tendencies  can  not  in  the  sun,  as  they  do  in  the  earth,  mask 
the  direct  and  obvious  effect  of  gravitation.  The  conse- 


ATOMS  AND  SUNBEAMS  19 

quence  is  that  the  intense  attraction  which  is  capable  of 
adding  velocity  to  the  molecules  at  the  phenomenal  rate  of 
four  hundred  and  fifty-seven  feet  per  second  is  permitted  to 
accomplish  something,  and  thus  increase  the  average  speeds 
with  which  the  molecules  hurry  along.  To  express  the 
matter  a  little  more  accurately,  we  should  say  that  the 
downward  velocity  imparted  by  gravitation,  being  com- 
pounded with  the  velocities  otherwise  possessed  by  the 
molecules,  tends,  on  the  whole,  to  increase  the  rate  at  which 
they  move. 

We  shall  now  be  able  to  discern  what  actually  takes 
place  as  the  sun  contracts  by  dispersing  heat,  and  in  con- 
sequence of  its  decline  in  bulk  finds  a  store  of  energy  lib- 
erated which  it  is  permitted  to  use  for  the  purpose  of  sus- 
taining its  radiating  capacity.  Owing  to  the  intense  heat 
which  prevails  in  the  photosphere,  the  molecules  must  there 
be  in  very  rapid  movement;  their  mutual  encounters  must 
be  of  the  utmost  vehemence,  and  their  internal  vibrations, 
which  are  the  consequences  of  the  shocks  in  the  encoun- 
ters, must  be  correspondingly  energetic.  It  is,  as  we  have 
seen,  these  internal  molecular  vibrations  which  set  the  ether 
in  motion,  and  thus  dispense  solar  heat  and  light  far  and 
wide  through  the  universe.  But  this  the  molecules  can 
only  do  at  the  expense  of  the  energy  which  they  possess 
in  virtue  of  their  internal  vibrations.  Unless,  therefore,  the 
internal  molecular  energy  were  to  be  in  some  way  recuper- 
ated from  time  to  time,  the  radiating  power  must  neces- 
sarily flag.  It  is  now  plain  that  the  necessary  recuperation 
takes  place  in  the  successive  encounters.  A  molecule 
whose  internal  energy  of  vibration  is  becoming  exhausted 
by  the  effort  of  setting  the  ether  into  vibration  presently 
impinges  against  some  other  molecule,  and  in  consequence 
of  the  blow  is  again  set  into  active  vibration  which  permits 
it  to  carry  on  the  work  of  radiation  anew,  until  its  declining 
energies  have  again  to  be  sustained  by  some  similar  addi- 
tion arising  from  a  fresh  collision.  Of  course,  we  know  that 
the  internal  molecular  energy  thus  acquired  can  not  be 
created  out  of  nothing.  If  the  molecule  receives  such  ac- 


20  BALL 

cessions  of  internal  energy,  it  must  be  at  the  expense  of 
the  energy  which  is  elsewhere.  Obviously  the  only  pos- 
sible source  of  such  energy  must  be  found  in  the  movement 
of  the  molecule  as  a  whole — that  is  to  say,  in  the  velocity 
of  translation  with  which  it  rushes  about  among  the  other 
molecules.  Thus  we  see  that  the  immediate  effect  of  ex- 
penditure of  heat  or  light  by  radiation  is  to  diminish  the 
internal  energies  of  the  molecules.  These  energies  are  re- 
stored by  the  transference  of  energy  obtained  from  the 
general  velocities  of  the  molecules  regarded  as  moving 
projectiles.  It  follows  that  the  velocities  of  the  several 
particles  must  on  the  whole  tend  to  decline;  in  other  words, 
that  the  temperature  tends  to  fall.  What  we  have  to  dis- 
cover is  the  agent  which  at  present  prevents  the  solar  tem- 
perature from  falling.  We  want,  therefore,  to  ascertain 
the  means  by  which  the  molecular  velocities  are  preserved 
at  the  same  average  value,  notwithstanding  that  there  is  a 
constant  tendency  for  these  velocities  to  abate  in  conse- 
quence of  the  losses  of  light  and  heat  by  radiation.  We 
have  already  explained  how  the  gravitation  of  the  sun  con- 
stantly tends  to  impart  additional  downward  velocity  to  the 
molecules  in  its  atmosphere.  This  is  precisely  the  action 
which  we  now  require.  The  contraction  of  the  sun  tends 
to  an  augmentation  of  the  molecular  velocities,  and  this 
augmentation  just  goes  to  supply  the  loss  of  velocities 
which  is  the  consequence  of  the  radiation.  A  complete  ex- 
planation of  the  maintenance  of  the  sun's  heat  is  thus 
afforded.  Observation,  no  doubt,  seems  to  show  that  the 
capacity  for  radiation  is  at  present  sensibly  constant,  and  this 
being  so,  we  see  that  the  gain  of  molecular  velocities  from 
gravitation  and  their  losses  from  radiation  are  at  present 
just  adapted  to  neutralize  each  other.  Nothing,  however, 
that  has  as  yet  been  said  demonstrates  that  the  efficiency 
of  the  sun  for  radiating  light  and  heat  must  always  be  pre- 
served exactly  at  its  present  value. 

It  is  quite  possible  that  if  we  had  the  means  of  studying 
the  sun's  heat  for  a  hundred  thousand  years,  we  might  find 
that  the  capacity  for  radiation  was  slightly  decreasing,  or, 


ATOMS  AND  SUNBEAMS  21 

it  may  be,  that  it  would  be  slightly  increasing,  for  it  is  at 
least  conceivable  that  the  gain  of  molecular  velocity  due 
to  gravitation  may,  on  the  whole,  exceed  the  loss  due  to 
the  dispersal  of  energy  by  radiation.  On  the  other  hand, 
it  is,  of  course,  possible  that  the  acquisition  of  velocity 
by  gravitation,  though  nearly  sufficient  to  countervail  the 
expenditure  by  radiation,  may  not  be  quite  enough,  in 
which  case  the  sun's  temperature  would  be  slowly  declining. 
It  must  not,  however,  be  supposed  that  the  argument 
which  we  have  been  here  following  attributes  eternal  vigour 
to  the  great  luminary.  It  will  be  noted  that  it  is  of  the 
essence  of  the  argument  that  the  contraction  is  still  in 
progress.  If  the  contraction  were  to  cease,  then  the  resti- 
tution of  velocity  by  gravitation  would  cease  also,  and  the 
speedy  dispersal  of  the  existing  heat  by  radiation  would 
presently  produce  bankruptcy  in  the  supply  of  sun- 
beams. Indeed,  such  bankruptcy  must  arrive  in  due  time, 
when,  after  certain  millions  of  years,  the  sun  has  so 
far  contracted  that  it  ceases  to  be  a  gaseous  mass.  The 
vast  accumulated  store  of  energy  which  is  now  being 
drawn  upon,  to  supply  the  current  radiation,  will  then  yield 
such  supplies  no  longer.  Once  this  state  has  been  reached, 
a  few  thousand  years  more  must  witness  the  extinction  of 
the  sun  altogether  as  a  source  of  light,  and  the  great  orb, 
at  present  our  splendid  luminary,  will  then  pass  over  into 
the  ranks  of  the  innumerable  host  of  bodies  which  were 
once  suns,  but  are  now  suns  no  longer. 


THE  WANDERINGS   OF  THE   NORTH 
POLE1 

ON  a  recent  visit  to  Cambridge,  Professor  Barnard, 
the  discoverer  of  the  fifth  satellite  of  Jupiter,  exhib- 
ited at  the  Cavendish  Laboratory  his  most  interest- 
ing collection  of  photographs  made  at  the  Lick  Observatory. 
These  pictures  were  obtained  by  a  six-inch  photographic 
lens  of  three  feet  focus,  attached  to  an  ordinary  equatorial, 
the  telescope  of  which  was  used  as  a  guider  when  it  was 
desired  to  obtain  a  picture  of  the  stars  with  a  long  exposure. 
Among  the  advantages  of  this  process  may  be  reckoned 
the  large  field  that  is  thereby  obtained,  many  of  the  plates 
that  he  exhibited  being  as  much  as  four  degrees  on  the 
edge.  I  am,  however,  not  now  going  to  speak  of  Barnard's 
marvellous  views  of  the  Milky  Way,  nor  of  the  plate  on 
which  a  comet  was  discovered,  nor  of  the  vicissitudes  of 
Holmes's  comet,  nor  of  that  wonderful  picture  in  which 
Swift's  comet  actually  appears  to  be  producing,  by  a  pro- 
cess of  gemmation,  an  offshoot  which  is  evidently  adapted 
for  an  independent  cometary  existence.  The  picture  to 
which  I  wish  specially  to  refer  in  connection  with  our 
immediate  subject  is  one  in  which  the  instrument  was 
directed  toward  the  celestial  pole.  In  this  particular  case 
the  clockwork  which  is  ordinarily  employed  to  keep  the 
stars  acting  at  the  same  point  of  the  plate  was  dispensed 
with.  The  telescope,  in  fact,  remained  fixed  while  the 
heavens  rotated  in  obedience  to  the  diurnal  motion.  Under 
these  circumstances  each  star,  as  minute  after  minute 
passed  by,  produced  an  image  on  a  different  part  of  the 
1  From  the  "  Fortnightly  Review,"  August,  1893. 

22 


WANDERINGS  OF  THE  NORTH   POLE  23 

plate;  the  consequence  of  which  was  that  the  record  which 
the  star  was  found  to  have  left,  when  the  picture  was  de- 
veloped, was  that  of  a  long  trail  instead  of  a  sharply  de- 
fined point.  As  each  star  appears  to  describe  a  circle  in  the 
sky  around  the  pole,  and  as,  in  the  vicinity  of  the  pole,  these 
circles  were  small  enough  to  be  included  in  the  plate,  this 
polar  photograph  exhibits  a  striking  spectacle.  It  dis- 
played a  large  number  of  concentric  circles,  or  rather,  I 
should  say,  of  portions  of  circles,  for  the  exposures  having 
lasted  for  about  four  hours,  about  one  sixth  of  each  circum- 
ference was  completed  during  that  time.  The  effect  thus 
produced  was  that  of  a  number  of  circular  arcs  of  varying 
sizes,  and  of  different  degrees  of  brightness.  Most  con- 
spicuous among  them  was  the  trail  produced  by  the  actual 
Pole  Star  itself.  It  is  well  known,  of  course,  that  though 
the  situation  of  the  pole  is  conveniently  marked  by  the  for-  , 
tunate  circumstance  that  a  bright  star  happened  during  the 
present  century  to  lie  in  the  immediate  vicinity  of  the 
veritable  pole,  yet,  of  course,  this  star  is  not  actually  at  the 
pole,  and  consequently,  like  all  the  other  stars,  Polaris  itself 
must  be  revolving  in  a  circle  whereof  the  centre  lies  at  the 
true  pole.  The  brighter  the  star  the  brighter  is  the  trail 
which  it  produces,  so  that  the  circle  made  by  Polaris  is 
much  more  conspicuous  than  the  circles  produced  by  the 
other  stars  of  inferior  lustre.  It  is,  however,  to  be  noted 
that  some  of  the  faint  stars  lie  much  closer  to  the  pole  than 
Polaris  itself.  There  is,  indeed,  one  very  minute  object  so 
close  to  the  pole  that  the  circle  in  which  its  movements 
are  performed  seems  very  little  more  than  a  point  when 
represented  on  the  screen  on  which  the  slide  was  projected. 
The  interesting  circumstance  was  noted  that  there  ap- 
peared to  be  occasional  interruptions  to  the  continuity  of 
the  circular  arcs.  This  was  due  to  the  fact  that  clouds  had 
interposed  during  the  intervals  represented  by  the  interrup- 
tions. A  practical  application  is  thus  suggested,  which  has 
been  made  to  render  useful  service  at  Harvard  College  Ob- 
servatory. Every  night,  and  all  night  long,  a  plate  is  there 
exposed  to  this  particular  part  of  the  sky,  and  the  degree 


24  BALL 

in  which  the  Pole  Star  leaves  a  more  or  less  complete  trail 
affords  an  indication  of  the  clearness  or  cloudiness  of  the 
sky  throughout  the  course  of  the  night.  From  the  posi- 
tions of  the  parts  where  the  trail  has  been  interrupted  it  is 
possible  not  only  to  learn  the  amount  of  cloudiness  that 
has  prevailed,  but  the  particular  hours  during  which  it  has 
lasted.  This  interesting  system  of  concentric  polar  circles 
affords  us  perhaps  the  most  striking  visual  representation 
that  could  possibly  be  obtained  of  the  existence  of  that 
point  in  the  heavens  which  we  know  as  the  pole.  The  pic- 
ture thus  exhibited  was  a  striking  illustration  of  the  Coper- 
nican  doctrine  that  the  diurnal  stellar  movement  was  indeed 
only  apparent,  being,  of  course,  due  to  the  rotation  of  the 
earth  on  its  axis. 

Suppose  that  a  photograph,  like  that  I  have  been  de- 
scribing, were  to  be  taken  at  intervals  of  a  century,  it  would 
be  found  that  the  centre  of  the  system  of  circles — that  is  to 
say,  the  veritable  pole  itself — was  gradually  changing  on 
the  heavens.  I  do  not  by  this  mean  that  the  stars  them- 
selves would  be  found  to  have  shifted  their  places  relatively 
to  each  other.  No  doubt  there  is  some  effect  of  this  kind, 
but  it  is  an  insignificant  one,  and  need  not  at  present  con- 
cern us.  The  essential  point  to  be  noticed  is,  that  the 
stars  which  happen  to  lie  in  the  vicinity  of  the  pole  would 
have  a  changed  relation  to  the  pole  in  consequence  of  the 
fact  that  this  latter  point  is  itself  in  incessant  movement. 
At  the  present  time  the  pole  is  advancing  in  such  a  direc- 
tion that  it  is  getting  nearer  to  the  Pole  Star,  so  that  the 
actual  circle  which  the  Pole  Star  is  describing  is  becoming 
less  and  less.  The  time  will  come  when  the  circle  which  this 
star  performs  will  have  reached  its  lowest  dimensions,  but 
still  the  pole  will  be  moving  on  its  way,  and  then,  of  course, 
the  dimensions  of  the  circle  traversed  by  the  Pole  Star  will 
undergo  a  corresponding  increase.  As  hundreds  of  years 
and  thousands  of  years  roll  by  the  pole  will  retreat  farther 
and  farther  from  the  Pole  Star,  so  that  in  the  course  of 
a  period  as  far  on  in  the  future  as  the  foundation  of  Rome 
was  in  the  past,  the  pole  will  be  no  longer  sufficiently  near 


WANDERINGS  OF  THE  NORTH  POLE       25 

the  Pole  Star  to  enable  the  latter  to  render  to  astronomers 
the  peculiar  services  which  it  does  at  present. 

Looking  still  further  ahead,  we  find  that  in  the  course 
of  about  twelve  thousand  years  the  pole  will  have  gained  a 
position  as  remote  as  it  possibly  can  from  that  position 
which  it  now  occupies.  This  most  critical  point  in  the 
heavens  will  then  lie  not  far  from  the  star  Vega,  the  bright- 
est point  in  the  northern  sky,  and  then  it  will  commence 
to  return,  so  that  after  the  lapse  of  about  twenty-five  thou- 
sand years  the  pole  will  be  found  again  in  the  same  celes- 
tial neighbourhood  where  it  is  to-night,  having,  in  the 
meantime,  traversed  a  mighty  circle  through  the  constel- 
lations. In  all  this  there  is  no  novelty;  these  movements 
of  the  pole  are  so  conspicuous  that  they  were  detected  long 
before  the  introduction  of  accurate  instruments.  They 
were  discovered  so  far  back  as  the  time  of  Hipparchus,  and 
the  cause  of  them  was  assigned  by  Newton  as  one  of  the 
triumphs  of  his  doctrine  of  universal  gravitation.  In  giv- 
ing the  title  of  "  The  Wanderings  of  the  North  Pole  "  to 
this  paper  I  did  not,  however,  intend  to  discourse  of  the 
movements  to  which  I  have  hitherto  referred.  They  are  so 
familiar  that  every  astronomer  has  to  attend  to  them  prac- 
tically in  the  reduction  of  almost  every  observation  of  the 
place  of  a  celestial  body.  It  was,  however,  necessary  to 
make  the  reference  which  I  have  done  to  this  subject  in 
order  that  the  argument  on  which  we  are  presently  to 
enter  should  be  made  sufficiently  clear.  It  must  be  noted 
that  the  expression  "  the  north  pole  "  is  ambiguous.  It 
may  mean  either  of  two  things,  which  are  quite  distinct. 
In  the  case  we  have  already  spoken  of,  I  understand  by  the 
north  pole  that  point  on  the  celestial  sphere  which  is  the 
centre  of  the  system  of  concentric  circles  described  by  the 
circumpolar  stars.  The  other  sense  in  which  the  north  pole 
is  used  is  the  terrestrial  one;  it  denotes  that  point  on  this 
earth  which  has  been  the  goal  of  so  many  expeditions,  and 
to  reach  which  has  been  the  ambition  of  so  many  illustrious 
navigators.  We  have  a  general  notion  that  the  terrestrial 
north  pole  lies  in  a  desolate  region  of  eternal  ice,  somewhat 


26  BALL 

relieved  by  the  circumstance  that  for  six  months  of  the 
year  the  frozen  prospect  is  brightened  by  perpetual  day, 
though,  on  the  other  hand,  during  the  remaining  six  months 
of  the  year  this  region  is  the  abode  of  perpetual  night. 
The  north  pole  is  that  hitherto  unattainable  point  on  our 
globe  on  which,  if  an  observer  could  take  his  station,  he 
would  find  that  the  phenomena  of  the  rising  and  the  set- 
ting of  the  stars,  so  familiar  elsewhere,  were  non-existent. 
Each  star  viewed  from  the  coign  of  vantage  offered  by  the 
north  pole  would  move  round  and  round  in  a  horizontal 
circle;  and  the  system  of  concentric  circles  would  be 
directly  overhead.  In  midsummer  the  sun  would  seem  to 
revolve  around,  remaining  practically  at  the  same  eleva- 
tion above  the  horizon  for  a  few  days,  until  it  slowly  began 
to  wend  its  way  downward  in  a  spiral.  In  a  couple  of 
months  it  would  draw  near  the  horizon,  and  as  day  after 
day  passed  by  the  luminary  would  descend  lower  and  lower 
until  its  edge  grazed  the  horizon  all  round.  The  setting 
of  the  sun  for  the  long  winter  would  then  be  about  to  com- 
mence, and  gradually  less  and  less  of  the  disk  would  re- 
main perceptible.  Finally,  the  sun  would  disappear  alto- 
gether, though  for  many  days  afterward  a  twilight  glow 
would  travel  round  the  whole  hemisphere,  ever  getting  less 
and  less,  until  at  last  all  indications  of  the  sun  had  vanished. 
The  utter  darkness  of  winter  would  then  ensue  for  months, 
mitigated  only  so  far  as  celestial  luminaries  were  concerned 
by  starlight  or  occasional  moonlight.  Doubtless,  however, 
the  fitful  gleams  of  the  aurora  would  often  suffice  to  render 
the  surrounding  desolation  visible.  Then  as  the  spring 
drew  near,  if,  indeed,  such  a  word  as  spring  be  at  all  ap- 
plicable to  an  abode  of  utter  dreariness,  a  faint  twilight 
would  be  just  discernible.  The  region  thus  illuminated 
would  move  round  and  round  the  horizon  each  twenty-four 
hours,  gradually  becoming  more  and  more  conspicuous, 
until  at  last  the  edge  of  the  sun  appeared.  Then,  by  a  spiral 
movement  inverse  to  that  with  which  its  descent  was  ac- 
complished, the  great  luminary  would  steal  above  the  hori- 
zon, there  to  continue  for  a  period  of  six  months  until  the 


WANDERINGS  OF   THE   NORTH   POLE  27 

beginning  of  the  ensuing  winter.  Indeed,  the  actual 
duration  of  apparent  summer  would  be  somewhat  pro- 
tracted in  consequence  of  the  effect  of  refraction  in  raising 
the  sun  visually  above  the  horizon  when  in  reality  it  was 
still  below.  The  result  would  be  to  lengthen  the  summer 
at  one  end  and  to  anticipate  it  at  the  other.  Such  would 
be  the  astronomical  conditions  at  the  north  pole;  that 
anomalous  point,  from  whence  every  other  locality  on  the 
globe  lies  due  south,  that  mysterious  point  which  up  to 
the  present  seems  never  to  have  been  approached  by  man 
within  a  distance  less  than  four  hundred  miles,  unless,  in- 
deed, as  is  not  improbably  the  case,  the  preglacial  man 
who  lived  in  the  last  genial  period  found  a  temperate  cli- 
mate, and  enjoyable  conditions  even  at  the  latitude  of  90°. 
For  our  present  purpose  it  will  be  necessary  to  get  a 
very  clear  idea  as  to  the  precise  point  on  the  earth  which  we 
mean  when  we  speak  of  the  north  pole.  As  our  knowledge 
of  it  is  almost  entirely  derived  from  astronomical  phe- 
nomena it  is  necessary  to  assign  the  exact  locality  of  the 
pole  by  a  strict  definition  depending  on  astronomical  facts. 
Supposing  that  Nansen  does  succeed  in  his  expedition, 
as  every  one  hopes  that  he  will,  and  does  penetrate  within 
that  circle  of  four  hundred  miles'  radius  where  the  foot  of 
civilized  man  has  never  yet  trod,  how  is  he  to  identify  that 
particular  spot  on  this  globe  which  is  to  be  defined  by  the 
north  pole?  It  was  for  this  purpose  that  at  the  begin- 
ning of  this  paper  I  referred  to  that  photograph  of  the  con- 
centric circles  which  illustrated  so  forcibly  the  position  of 
the  pole  in  the  heavens.  Imagine  that  your  eye  was  placed 
at  the  centre  of  the  earth,  and  that  you  had  a  long,  slender 
tube  from  that  centre  to  the  surface  through  which  you 
could  look  out  at  the  celestial  sphere;  if  that  tube  be  placed 
in  such  a  way  that,  when  looking  from  the  centre  of  the 
earth  through  this  tube  your  vision  was  directed  exactly  to 
that  particular  point  of  the  heavens  which  is  the  centre  of 
the  circle  now  described  by  the  Pole  Star  and  the  other 
circumpolar  stars,  then  that  spot  in  which  the  end  of  the 
tube  passes  out  through  the  surface  of  the  earth  is  the  north 


28  BALL 

pole.  Imagine  a  stake  to  be  driven  into  the  earth  at  the 
place  named,  then  the  position  of  that  stake  is  the  critical 
spot  on  our  globe  which  has  been  the  object  of  so  much  sci- 
entific investigation  and  of  so  much  maritime  enterprise. 
The  reader  must  not  think  that  I  am  attempting  to  be  hy- 
peraccurate  in  this  definition  of  the  north  pole;  no  doubt,  in 
our  ordinary  language  we  often  think  of  the  pole  as  some- 
thing synonymous  with  the  polar  regions,  an  ill-defined 
and  most  vaguely  known  wilderness  of  ice.  For  scientific 
purposes  it  is,  however,  essential  to  understand  that  the  pole 
is  a  very  definitely  marked  point,  and  we  must  assign  its 
position  accurately,  not  merely  to  within  miles,  but  even  to 
within  feet.  Indeed,  it  is  a  truly  extraordinary  circum- 
stance that,  considering  no  one,  with  the  possible  excep- 
tion just  referred  to,  has  ever  yet  been  within  so  many 
hundreds  of  miles  of  the  pole,  we  should  be  able  to  locate 
it  so  precisely  that  we  are  absolutely  certain  of  its  position 
to  within  an  area  not  larger  than  that  covered  by  a  small 
town,  or  even  by  a  good-sized  drawing-room. 

We  have  seen  that  the  north  pole  in  the  sky  is  in  inces- 
sant movement,  and  that  the  travels  which  it  accomplishes 
in  the  course  of  many  centuries  extend  over  a  wide  sweep 
of  the  heavens;  this  naturally  suggests  the  question,  Does 
the  pole  in  the  earth  move  about  in  any  similar  manner, 
and  if  so,  what  are  the  nature  and  extent  of  its  variation? 
Here  is  the  point  about  which  those  modern  researches 
have  been  made  which  it  is  my  special  object  to  discuss 
in  this  paper.  Let  us  first  see  clearly  the  issue  that  is 
raised.  At  the  time  of  the  building  of  the  Pyramids  the 
pole  in  the  heavens  was  in  quite  a  different  place  from  its 
present  position;  the  Pole  Star  had  not  at  that  time  the 
slightest  title  to  be  called  a  pole  star;  in  fact,  the  point 
around  which  the  heavens  revolved  lay  in  a  wholly  differ- 
ent constellation.  It  was  certainly  not  far  from  the  star 
Alpha  Draconis  about  3000  B.  c.,  and  we  could  indicate  its 
position  quite  definitely  if  we  had  any  exact  knowledge  as 
to  the  date  of  the  Pyramids'  erection.  It  is,  however,  plain 
that  the  difference  was  so  patent  between  the  celestial  pole 


WANDERINGS  OF  THE  NORTH  POLE       29 

at  the  time  of  the  Pyramids  and  the  celestial  pole  of  later 
centuries,  that  it  could  not  be  overlooked  in  attentive  ob- 
servation of  the  heavens.  As  the  north  pole  in  the  sky 
was,  therefore,  so  different  in  the  time  of  the  Pharaohs  from 
the  north  pole  in  the  time  of  Victoria,  it  is  proper  to  ask 
whether  there  was  a  like  difference,  or  any  difference  at  all, 
between  the  terrestrial  pole  at  the  time  of  the  building  of 
the  Pyramids  and  that  terrestrial  pole  in  whose  quest  Nan- 
sen  is  just  setting  off.  If  Pharaoh  had  despatched  a  suc- 
cessful expedition  to  the  north  pole  and  driven  a  post  in 
there  to  mark  it,  and  if  Nansen  were  now  successful,  would 
he  find  that  the  north  pole  in  the  earth  which  he  was  to 
mark  occupied  the  same  position  or  a  different  position 
from  that  which  had  been  discovered  thousands  of  years 
previously?  At  first  one  might  hastily  say  that  there  must 
be  such  a  difference,  for  it  will  be  remembered  that  I  have 
defined  the  north  pole  in  the  earth  as  that  point  through 
which  the  tube  passes  which  would  permit  an  eye  placed  at 
the  centre  of  the  earth  to  view  the  north  pole  in  the  sky. 
If,  therefore,  the  north  pole  in  the  sky  had  undergone  a 
great  change  in  its  position,  it  might  seem  obvious  that  the 
tube  from  the  earth's  centre  to  its  surface  which  would  now 
conduct  the  vision  from  that  centre  to  the  north  celestial 
pole  would  emerge  at  a  different  point  of  the  earth's  crust 
from  that  which  it  formerly  occupied.  We  have  here  to  deal 
with  the  case  that  arises  not  unfrequently  in  astronomy,  in 
which  a  fact  of  broad  general  truth  requires  a  minute  degree 
of  qualification;  indeed,  it  is  not  too  much  to  say  that  it 
is  in  this  qualification  of  broad  general  truths  that  many  of 
the  greatest  discoveries  in  physical  science  have  consisted. 
And  such  is  the  case  in  the  present  instance.  There  is  a 
broad  general  truth  and  there  is  the  qualification  of  it.  It 
is  the  qualification  that  constitutes  the  essential  discovery 
which  it  is  my  object  herein  to  set  forth.  But  before  doing 
so  it  will  be  necessary  for  me  to  lay  down  the  broad  general 
truth  that  the  north  pole  of  the  earth  as  it  existed  in  the 
time  of  the  Pharaohs  appears  to  be  practically  the  same  as 
the  north  pole  of  the  earth  now.  It  seems  perfectly  certain 


30  BALL 

that  at  any  time  within  the  last  ten  thousand  years  the 
north  pole  might  have  been  found  within  a  region  on  the 
earth's  surface  not  larger  than  Hyde  Park.  Indeed,  the 
limits  might  be  drawn  much  more  closely.  It  is  quite  pos- 
sible that  many  an  edifice  in  London  occupies  an  area  suf- 
ficiently great  to  cover  the  holes  that  would  be  made  by 
all  the  posts  that  might  be  driven  to  mark  the  precise  sites 
of  the  north  pole  on  the  earth  not  only  for  the  last  five 
thousand  or  ten  thousand  years,  but  probably  for  periods 
much  more  ancient  still.  It  is  very  likely  that  the  north 
pole  at  the  time  of  the  Glacial  epoch  was  practically  in- 
distinguishable from  the  north  pole  now;  in  fact,  the  con- 
stancy, or  sensible  constancy  I  should,  perhaps,  rather 
say,  of  the  situation  of  this  most  critical  point  in  our  globe 
is  one  of  the  most  astonishing  facts  in  terrestrial  physics. 

Let  us,  then,  assume  this  broad,  general  fact  of  the  per- 
manency in  the  position  of  the  north  pole,  and  deduce  the 
obvious  consequences  it  implies  with  regard  to  the  earth's 
movement.  At  this  point  we  find  the  convenience  of  the 
time-honoured  illustration  in  our  geography  books  which 
likens  the  earth  to  an  orange.  Let  us  thrust  a  knitting- 
needle  through  the  orange  along  its  shortest  diameter  to 
represent  the  axis  about  which  the  earth  rotates.  Not  only 
does  the  earth  perform  one  revolution  about  this  axis  in 
the  space  of  each  sidereal  day,  but  the  axis  itself  has  a 
movement.  If  the  earth's  axis  always  remained  fixed,  or 
never  had  any  motion  except  in  a  direction  parallel  to  itself, 
then  the  point  to  which  it  was  directed  on  the  sky  wotild 
never  change.  We  have,  however,  seen  that  the  pole  in 
the  sky  is  incessantly  altering  its  position;  we  are  therefore 
taught  that  the  direction  of  the  earth's  axis  of  rotation  is 
constantly  changing.  To  simulate  the  movement  by  the 
orange  and  knitting-needle  we  must  imagine  the  orange  to 
rotate  around  its  axis  once  in  that  period  of  twenty-three 
hours  and  fifty-six  minutes,  which  is  well  known  as  the 
length  of  the  sidereal  day;  while  at  the  same  time  the  knit- 
ting-needle, itself  bearing,  of  course,  the  orange  with  it, 
performs  a  conical  movement  with  such  extreme  slowness 


WANDERINGS  OF  THE   NORTH  POLE  31 

that  not  less  than  twenty-five  thousand  years  is  occupied 
in  making  the  circuit.  The  movement,  as  has  often  been 
pointed  out,  is  like  that  of  a  peg-top  which  rotates  rapidly 
on  its  axis  while  at  the  same  time  the  axis  itself  has  a  slow 
revolving  motion.  Thus  the  phenomena  which  are  pre- 
sented in  the  rotation  of  the  earth  demonstrate  that  the 
axis  about  which  the  earth  rotates  occupies  what  is,  at  all 
events,  approximately  a  fixed  position  in  the  earth,  though 
not  a  fixed  position  in  space.  We  can  hardly  be  surprised 
at  this  result;  it  merely  implies  that  the  earth  acts  like  a 
rigid  body  on  the  whole,  and  does  not  permit  the  axis 
about  which  it  is  turning  to  change  its  position. 

It  will  now  be  easily  understood  how  it  comes  to  pass 
that  the  position  of  the  north  pole  upon  the  earth  has  not 
appreciably  changed  in  the  course  of  thousands  of  years. 
The  axis  around  which  the  earth  rotates  has  retained  a  per- 
manent position  relative  to  the  earth  itself;  it  has,  however, 
continuously  changed,  it  is  at  this  moment  changing,  and  it 
will  continue  to  change  with  regard  to  its  direction  in  space. 
So  far  our  knowledge  extended  up  to  within  the  last  few 
years,  but  in  these  modern  days  a  closer  inquiry  has  been 
made  into  this,  as  into  so  many  other  physical  subjects,  and 
the  result  has  been  to  disclose  the  important  fact  that, 
though  the  phenomena  as  just  described  are  very  nearly 
true,  they  must  receive  a  certain  minute  qualification. 
Complete  examination  of  this  subject  is  desirable,  not  only 
on  account  of  its  natural  importance,  but  also  because 
it  illustrates  the  refinements  of  which  modern  astronomical 
processes  are  susceptible.  I  have  stated  the  broad,  general 
fact  that  the  position  of  the  terrestrial  pole  undergoes  no 
large  or  considerable  fluctuation.  But  while  we  admit  that 
no  large  fluctuation  is  possible,  it  is  yet  very  proper  to 
consider  whether  there  may  not  be  a  small  fluctuation.  It 
is  certain  that  the  position  of  the  pole  as  it  would  be 
marked  by  a  post  driven  into  the  earth  to-day  can  not  differ 
by  a  mile  from  the  position  in  which  the  same  point  would 
be  marked  last  year  or  next  year.  But  does  it  differ  at  all? 
Is  it  absolutely  exactly  the  same?  Would  there  be  a  dif- 


32  BALL 

ference  not  indeed  of  miles,  but  of  yards  or  of  feet,  between 
the  precise  position  of  the  pole  on  the  earth  determined 
at  successive  intervals  of  time?  Would  it  be  the  same  if 
we  carried  out  our  comparisons  not  merely  between  one 
year  and  another,  but  day  after  day,  week  after  week, 
month  after  month?  No  doubt  the  more  obvious  phe- 
nomena proclaim  in  the  most  unmistakable  manner  that 
the  position  of  the  pole  is  substantially  invariable.  If, 
therefore,  there  be  any  fluctuations  in  its  positions,  those 
could  only  be  disclosed  by  careful  scrutiny  of  minute  phe- 
nomena which  were  too  delicate  to  be  detected  in  the 
coarser  methods  of  observation.  There  is  indeed  a  cer- 
tain presumption  in  favour  of  the  notion  that  absolute  con- 
stancy in  the  position  of  the  pole  need  not  be  expected. 
Almost  every  statement  of  astronomical  doctrine  requires 
its  qualification,  and  it  would  seem  indeed  unlikely  that 
when  sufficient  refinement  was  introduced  into  the  meas- 
urements the  position  of  the  pole  in  the  earth  should  appear 
to  be  absolutely  unalterable.  Until  a  very  recent  period  the 
evidence  on  the  subject  was  almost  altogether  negative;  it 
was  no  doubt  recognised  that  there  might  be  some  fluctua- 
tions in  the  position  of  the  pole,  but  it  was  known  that 
they  would  only  be  extremely  small,  and  it  was  believed 
that  in  all  probability  those  fluctuations  must  be  comprised 
within  those  slender  limits  which  are  too  much  affected 
by  inevitable  errors  of  observation  to  afford  any  reliable 
result.  Perseverance  in  this  interesting  inquiry  has  been 
at  last  rewarded;  and,  as  in  so  many  similar  cases,  we  are 
indebted  to  the  labours  of  many  independent  workers  for 
the  recent  extension  of  our  knowledge.  We  are,  however, 
at  present  most  interested  by  the  labours  of  Mr.  Chandler, 
a  distinguished  American  astronomer,  who  has  made  an 
exhaustive  examination  into  the  subject.  The  result  has 
been  to  afford  a  conclusive  proof  that  the  terrestrial  pole 
does  undergo  movement.  Mr.  Chandler  has  been  so  suc- 
cessful as  to  have  determined  the  law  of  those  polar  move- 
ments, and  he  has  found  that  when  they  are  taken  into  con- 
sideration an  important  improvement  in  certain  delicate 


WANDERINGS   OF  THE   NORTH   POLE  33 

astronomical  inquiries  is  the  result.  These  valuable  in- 
vestigations merit,  in  the  highest  degree,  the  attention,  not 
only  of  those  who  are  specially  devoted  to  astronomical 
and  mathematical  researches,  but  of  that  large  and  ever- 
increasing  class  who  are  anxious  for  general  knowledge 
with  regard  to  the  physical  phenomena  of  our  globe. 

At  first  sight  it  might  seem  difficult  indeed  to  conduct 
the  investigation  of  this  question.  Here  is  a  point  on  the 
earth's  surface,  this  wonderful  north  pole,  which,  so  far  as 
we  certainly  know,  has  never  yet  been  approached  to  within 
four  hundred  miles,  and  yet  we  are  so  solicitous  about  the 
position  of  this  pole  and  about  its  movement  that  we  de- 
mand a  knowledge  of  its  whereabouts  with  an  accuracy 
which  at  first  apears  wholly  unattainable.  It  sounds  almost 
incredible  when  we  are  told  that  a  shift  in  the  position  of 
the  north  pole  to  the  extent  of  twenty  yards,  or  even  of 
twenty  feet,  is  appreciable,  notwithstanding  that  we  have 
never  been  able  to  get  nearer  to  it  than  from  one  end  of 
England  to  the  other.  Indeed,  as  a  matter  of  fact,  our 
knowledge  of  the  movements  of  the  pole  are  derived  from 
observations  made  not  alone  hundreds  but  even  many 
thousands  of  miles  distant.  It  is  in  such  observatories  as 
those  at  Greenwich  or  Berlin,  Pulkowa  or  Washington, 
that  the  determinations  have  been  made  by  which  changes 
in  the  position  of  the  pole  can  be  ascertained  with  a  deli- 
cacy and  precision  for  which  those  would  hardly  be  pre- 
pared who  were  not  aware  of  the  refinement  of  modern 
astronomical  methods.  I  do  not,  however,  imply  that  the 
observations  conducting  to  the  discoveries  now  about  to 
be  considered  have  been  exclusively  obtained  at  the  ob- 
servatories I  have  named.  There  are  a  large  number  of 
similar  institutions  over  the  globe  which  have  been  made 
to  bear  their  testimony.  Tens  of  thousands  of  different 
observations  have  been  brought  together,  and  by  discuss- 
ing them  it  has  been  found  possible  to  remove  a  large  part 
of  the  errors  by  which  such  work  is  necessarily  affected, 
and  to  elicit  from  the  vast  mass  those  grains  of  truth  which 
could  not  have  been  discovered  had  it  not  been  for  the 


34  BALL 

enormous  amount  of  material  that  was  available.  Mr. 
Chandler  has  discussed  these  matters  in  a  remarkable  series 
of  papers,  and  it  will  be  necessary  for  me  now  to  enter  into 
some  little  detail,  both  as  regards  the  kind  of  observations 
that  have  been  made,  and  the  results  to  which  astronomers 
have  been  thereby  conducted. 

Greenwich  Observatory  lies  more  than  two  thousand 
miles  from  the  north  pole,  and  yet  if  the  pole  were  to  shift 
by  as  much  as  the  width  of  Regent  Street,  the  fact  that  it 
had  done  so  would  be  quite  perceptible  at  Greenwich.  Let 
me  endeavour  to  explain  how  such  a  measurement  could 
be  achieved.  In  finding  the  latitude  at  any  locality  we 
desire,  of  course,  to  know  the  distance  between  the  locality 
and  the  equator,  expressed  in  angular  magnitude.  But 
though  this  is  distinctly  the  definition  of  latitude,  it  does 
not  at  once  convey  the  idea  as  to  how  this  element  can  be 
ascertained.  How,  for  instance,  would  an  astronomer  at 
Greenwich  be  able  to  learn  the  angular  distance  of  the 
observatory  from  the  equator?  The  equator  is  not  marked 
on  the  sky,  and  it  is  obvious  that  the  observer  must  employ 
a  somewhat  indirect  process  to  ascertain  what  he  wants. 
Here,  again,  we  have  to  invoke  the  aid  of  that  celestial 
pole  to  which  I  have  so  often  referred.  Think  of  that  point 
on  the  sky  which  is  the  common  centre  of  the  circles  ex- 
hibited on  Professor  Barnard's  photograph.  That  point 
is  not  indeed  marked  by  any  special  star,  but  it  is  com- 
pletely defined  by  the  circumstance  that  it  is  the  centre  of 
the  track  performed  by  the  circumpolar  stars.  We  thus 
obtain  a  clear  idea  of  this  definite  point  in  the  sky,  and 
the  horizon  is  a  perfectly  definite  line,  at  all  events  from 
any  station  where  the  sea  is  visible.  It  is  not  difficult  to 
imagine  that  by  suitable  measurements  we  can  ascertain 
the  altitude  of  this  point  in  the  heavens  above  the  horizon. 
That  altitude  is  the  latitude  of  the  place;  it  is,  in  fact,  the 
very  angle  which  lies  between  the  locality  on  the  earth  and 
the  equator.  It  is  quite  true  that  as  the  pole  is  implied  by 
these  circles  rather  than  directly  marked  by  them,  the  meas- 
urement of  the  altitude  can  not  be  effected  quite  directly. 


WANDERINGS  OF  THE  NORTH  POLE       35 

The  actual  process  is  to  take  the  Pole  Star,  or  some  one  of 
the  other  circumpolar  stars,  and  to  measure  the  greatest 
height  to  which  it  ascends  above  the  horizon  and  the  lowest 
altitude  to  which  it  declines  about  twelve  hours  later.  The 
former  of  these  is  as  much  above  the  pole  as  the  latter  is 
below  it,  so  between  them  we  are  able  to  ascertain  the 
altitude  of  the  pole  with  a  high  degree  of  accuracy.  It  is 
true  that  in  a  fixed  observatory  such  as  Greenwich  there  is 
no  visible  sea  horizon,  and  even  if  there  were  it  would  not 
provide  so  excellent  a  method  as  is  offered  by  the  equiva- 
lent process  of  first  observing  the  star  directly  and  then 
observing  its  reflection  from  a  dish  of  mercury.  In  this 
way  the  altitude  of  the  star  above  the  horizon  is  determined 
with  the  utmost  precision.  The  practical  astronomer  will, 
however,  remember  that,  of  course,  he  has  to  attend  to  the 
effects  of  atmospheric  refraction,  which  invariably  shows  a 
star  higher  up  than  it  ought  to  be.  This  can  be  allowed 
for,  and  in  this  way  the  latitude  of  the  observatory  is  ascer- 
tained with  all  needful  accuracy.  When  the  highest  degree 
of  precision  is  sought  for,  and  it  is  only  observations  with 
a  very  high  degree  of  precision  which  are  available  for  our 
present  purpose,  a  considerable  number  of  stars  have  to  be 
employed,  and  very  many  observations  have  to  be  taken 
at  different  seasons  of  the  year,  so  as  to  eliminate  as  far  as 
possible  all  sources  of  casual  error.  When,  however,  due 
attention  has  been  paid  to  those  precautions  which  the 
experience  of  astronomers  suggests,  the  result  that  is  ob- 
tained is  characterized  by  extraordinary  precision.  How 
great  that  precision  may  be  I  must  endeavour  to  ex- 
plain. The  latitude  of  every  important  observatory  is 
obtained  from  a  large  number  of  observations,  and  it 
would  be  unlikely  that  it  was  more  than  one  or  two 
tenths  of  a  second  different  from  the  actual  mean  value. 
Now,  a  tenth  of  a  second  on  the  surface  of  the  earth  cor- 
responds to  a  distance  of  about  ten  feet,  and  this  means 
that  the  latitude  of  the  observatory  or,  as  we  must  now 
speak  very  precisely,  the  latitude  of  the  centre  of  the 
meridian  circle  in  the  observatory,  is  known  to  a  degree 


36  BALL 

of  precision  represented  by  a  few  paces.  It  will  thus  be 
seen  that,  with  the  accuracy  attainable  in  our  modern  ob- 
servations, it  would  often  be  an  appreciable  blunder  to  mis- 
take the  latitude  of  one  wall  of  the  observatory  for  that 
of  the  opposite  wall;  in  other  words,  we  know  accurately  to 
within  the  tenth  of  a  second,  or  within  not  much  more 
than  the  tenth  of  a  second,  the  distance  from  the  centre  of 
the  transit  circle  at  Greenwich  down  to  the  earth's  equator. 
But,  of  course,  the  distance  from  the  pole  to  the  equator  is 
90°,  and  this  being  so,  it  follows  that  the  distance  from  the 
north  pole  of  the  earth  to  the  centre  of  the  transit  circle 
at  Greenwich  Observatory  has  been  accurately  ascertained 
to  within  one  or  two  tenths  of  a  second.  If  any  change 
took  place  in  the  distance  between  the  pole  and  the 
meridian  circle  at  Greenwich,  then  it  must  be  manifested 
by  the  changes  of  latitude.  We  shall  now  be  able  to  under- 
stand how  any  movement  of  the  pole,  or  rather  of  the  posi- 
tion which  it  occupies  in  the  earth,  would  be  indicated  at 
Greenwich.  Suppose,  for  instance,  that  the  pole  actually 
advanced  toward  Great  Britain,  and  that  it  moved  to  a  dis- 
tance of,  let  us  say,  thirty  feet,  the  effect  of  this  would  be 
to  produce  a  diminution  of  the  distance  between  the  pole 
and  Greenwich — that  is  to  say,  there  must  be  an  increase 
in  the  distance  from  Greenwich  to  the  equator.  This  would 
correspond  to  a  change  in  the  latitude  of  Greenwich;  that 
latitude  would  diminish  by  three  tenths  of  a  second,  which 
is  a  magnitude  quite  large  enough  to  be  recognisable  by 
the  observations  I  have  already  indicated  as  proper  for 
the  determination  of  latitude.  A  shift  of  the  pole  to  a 
distance  of  sixty  feet  would  be  a  conspicuous  alteration 
announced  in  every  observatory  in  Europe  provided  with 
instruments  of  good  modern  construction. 

Until  the  last  few  years  there  was  not  much  reason  to 
think  that  the  pole  exhibited  any  unequivocal  indications 
of  movement.  No  doubt,  displacements  resembling  those 
which  have  now  been  definitely  ascertained  have  existed 
for  many  years,  but  they  were  too  small  to  produce  any 
appreciable  effect,  except  with  instruments  of  a  more  re- 


WANDERINGS  OF  THE  NORTH  POLE       37 

fined  description  than  those  with  which  the  earlier  observa- 
tories were  equipped.  It  was  obvious  that  the  pole  did 
not  make  movements  of  anything  like  a  hundred  yards  in 
extent;  had  it  done  so  the  resulting  variations  in  latitude 
would  have  been  conspicuous  enough  to  have  obtained 
notice  many  years  ago.  The  actual  movements  which  the 
pole  does  make  are  of  that  small  character  which  require 
very  minute  discussion  of  the  observations  to  establish 
them  beyond  reach  of  cavil.  There  is,  however,  one  striking 
method  of  confirming  such  observations  as  have  been  made 
which  leaves  no  doubt  of  the  accuracy  of  the  results  to 
which  they  point.  Suppose,  for  instance,  that  the  great  ob- 
servatories in  Europe  indicate  at  a  certain  time  that  their 
latitudes  have  all  increased;  this  necessarily  implies  that  the 
equator  has  receded  from  them,  and  that,  therefore,  the 
north  pole  has  approached  Europe.  If,  however,  the  north 
pole  has  approached  Europe  it  must  have  retreated  from 
those  regions  on  the  opposite  side  of  the  world — say,  for 
instance,  the  Sandwich  Islands.  Observations  in  the  Sand- 
wich Islands  should,  therefore,  indicate,  if  our  reasoning 
has  been  correct,  that  the  pole  has  retreated  from  them, 
and  that  the  equator  has,  therefore,  advanced  in  such  a 
way  that  the  latitudes  of  localities  in  the  Sandwich  Islands 
have  diminished.  The  various  observations  which  have 
been  brought  together  by  the  diligence  of  Mr.  Chandler, 
including  those  which  he  has  himself  made  with  an  in- 
genious apparatus  of  his  own  design,  have  been  submitted 
to  this  test,  and  they  have  borne  it  well.  The  result  has 
been  that  it  is  now  possible  to  follow  the  movements  of 
the  pole  with  a  considerable  degree  of  completeness.  Pro- 
fessor Chandler  has  tracked  the  pole  month  after  month, 
year  after  year,  through  a  period  of  more  than  a  century 
of  exact  observations,  and  he  has  succeeded  in  determining 
the  movements  which  this  point  undergoes.  Let  me  here 
endeavour  to  describe  the  result  at  which  he  has  arrived. 

In  that  palaeocry stic  ocean  which  arctic  travellers  have 
described,  where  the  masses  of  ice  lie  heaped  together  in 
the  wildest  confusion,  lies  this  point  which  is  the  object 


38  BALL 

of  so  much  speculation.  Let  us  think  of  this  tract,  or  a 
portion  of  it,  to  be  levelled  to  a  plain,  and  at  a  particular 
centre  let  a  circle  be  drawn  the  radius  of  which  is  about 
thirty  feet;  it  is  in  the  circumference  of  this  circle  that  the 
pole  of  the  earth  is  constantly  to  be  found.  In  fact,  if  at 
different  times,  month  after  month  and  year  after  year,  the 
position  of  the  pole  was  ascertained  as  the  extremity  of 
that  tube  from  which  an  eye  placed  at  the  centre  of  the 
earth  would  be  able  to  see  the  pole  of  the  heavens,  and  if 
the  successive  positions  of  this  pole  were  marked  by  pegs 
driven  into  the  ground,  then  the  several  positions  in  which 
the  pole  would  be  found  must  necessarily  trace  out  the 
circumference  of  the  circle  that  has  been  thus  described. 
The  period  in  which  each  revolution  of  the  pole  around 
the  circle  takes  place  is  about  four  hundred  and  twenty- 
seven  days;  the  result,  therefore,  of  these  investigations 
shows,  when  the  observations  are  accurate,  that  the  north 
pole  of  the  earth  is  not,  as  has  been  so  long  supposed,  a 
fixed  point,  but  that  it  revolves  around  in  the  earth,  ac- 
complishing each  revolution  in  about  two  months  more 
than  the  period  that  the  earth  requires  for  the  performance 
of  each  revolution  around  the  sun. 

The  discovery  of  the  movement  of  the  pole  which  I 
have  here  described  must  be  regarded  as  a  noteworthy 
achievement  in  astronomy,  nor  is  the  result  to  which  it 
leads  solely  of  interest  in  consequence  of  the  lesson  it 
teaches  us  with  regard  to  the  circumstances  of  the  earth's 
rotation.  It  has  a  higher  utility,  which  the  practical 
astronomer  will  not  be  slow  to  appreciate,  and  of  which  he 
has,  indeed,  already  experienced  the  benefit.  There  are 
several  astronomical  investigations  in  which  the  latitude 
of  the  observatory  enters  as  a  significant  element.  Lati- 
tude is,  in  fact,  at  every  moment  employed  as  an  important 
factor  in  many  astronomical  determinations:  to  take  one 
of  the  most  simple  cases,  suppose  that  we  are  finding  the 
place  of  a  planet  in  the  observatory,  we  deduce  its  posi- 
tion by  measuring  its  zenith  distance,  and  then  to  obtain 
the  declination  the  latitude  of  the  observatory  has,  of 


WANDERINGS   OF   THE   NORTH   POLE  39 

course,  to  be  considered.  Now,  astronomers  have  hitherto 
been  in  the  habit  of  accepting  the  determination  of  their 
latitude  which  had  been  established  by  a  protracted  series 
of  observations,  and  treating  it  as  if  it  were  'a  constant. 
This  method  will  be  no  longer  admissible  when  astro- 
nomical work  of  the  highest  class  is  demanded.  No  doubt, 
from  the  sailor's  point  of  view,  an  alteration  in  latitude 
which  at  most  amounts  to  a  shift  of  sixty  feet,  not  a  quarter, 
perhaps,  of  the  length  of  his  vessel,  is  immaterial.  But  in 
the  more  refined  parts  of  astronomical  work  these  discov- 
eries can  no  longer  be  overlooked;  indeed,  Mr.  Chandler 
has  shown  that  many  discrepancies  by  which  astronomers 
had  been  baffled  can  be  removed  when  note  is  taken  of 
the  circumstance  that  the  latitude  of  the  observatory  is  in 
an  incessant  condition  of  transformation  in  accordance  with 
the  law  which  his  labours  have  expounded.  It  will  ere 
long  be  necessary  in  every  observatory  where  important 
work  is  being  done  to  obtain  for  every  day  the  correction 
to  the  mean  value  of  the  latitude,  in  order  to  obtain  the 
value  appropriate  for  that  day. 

There  are  also  other  grounds  of  a  somewhat  profounder 
character  on  which  the  discoveries  now  made  are  eminently 
instructive.  Those  who  are  interested  in  the  physics  of  our 
globe  often  discuss  the  question  as  to  whether  the  internal 
heat,  which  the  earth  certainly  possesses,  is  sufficiently  in- 
tense to  render  the  deep-seated  portions  of  our  globe  more 
or  less  fluid.  On  the  other  hand,  the  effects  of  pressure, 
especially  of  such  pressures  as  are  experienced  in  the 
depths  hundreds  and  thousands  of  miles  below  the  surface, 
must  go  far  to  consolidate  the  materials  to  form  what  must 
be  sensibly  a  rigid  body.  The  question,  therefore,  arises, 
Is  the  earth  to  be  regarded  as  a  rigid  mass,  or  is  it  not? 
The  phenomena  of  the  tides  had  already  to  some  extent 
afforded  information  on  this  subject,  and  now  Mr.  Chan- 
dler's investigation  adds  much  further  light,  for  it  is  certain 
from  his  result  that  the  earth  can  not  be  a  rigid  body.  It 
is  quite  true  that,  even  though  the  earth  were  rigid,  the 
pole  might  go  round  in  a  circle,  and  that  circle  might  have 


40  BALL 

a  thirty-feet  radius,  but  in  such  a  case  the  period  would  be 
only  about  three  quarters  of  the  four  hundred  and  twenty- 
seven  days  which  he  has  found.  In  the  interest,  therefore, 
of  the  theoretical  astronomer,  as  well  as  on  the  other 
grounds  which  I  have  set  forth,  Mr.  Chandler's  investiga- 
tions must  be  regarded  as  a  most  important  contribution 
to  modern  astronomy. 


THE  AGE  OF  THE  SUN'S  HEAT 

BY 

SIR  WILLIAM  THOMSON   (LORD   KELVIN) 


THE  AGE  OF  THE  SUN'S  HEAT1 

THE  second  great  law  of  thermodynamics  involves  a 
certain  principle  of  irreversible  action  in  Nature.  It 
is  thus  shown  that,  although  mechanical  energy  is 
indestructible,  there  is  a  universal  tendency  to  its  dissipa- 
tion, which  produces  gradual  augmentation  and  diffusion 
of  heat,  cessation  of  motion,  and  exhaustion  of  potential 
energy  through  the  material  universe.2  The  result  would 
inevitably  be  a  state  of  universal  rest  and  death,  if  the  uni- 
verse were  finite  and  left  to  obey  existing  laws.  But  it  is 
impossible  to  conceive  a  limit  to  the  extent  of  matter  in 
the  universe;  and  therefore  science  points  rather  to  an 
endless  progress,  through  an  endless  space,  of  action  in- 
volving the  transformation  of  potential  energy  into  pal- 
pable motion  and  thence  into  heat,  than  to  a  single  finite 
mechanism,  running  down  like  a  clock,  and  stopping  for- 
ever. It  is  also  impossible  to  conceive  either  the  begin- 
ning or  the  continuance  of  life,  without  an  overruling 
creative  power;  and,  therefore,  no  conclusions  of  dynam- 
ical science  regarding  the  future  condition  of  the  earth  can 
be  held  to  give  dispiriting  views  as  to  the  destiny  of  the 
race  of  intelligent  beings  by  which  it  is  at  present  inhabited. 
The  object  proposed  in  the  present  article  is  an  ap- 
plication of  these  general  principles  to  the  discovery  of 
probable  limits  to  the  periods  of  time,  past  and  future, 
during  which  the  sun  can  be  reckoned  on  as  a  source  of 

1  From  "  Macmillan's  Magazine,"  March,  1862. 

3  See  "  On  a  Universal  Tendency  in  Nature  to  the  Dissipation  of 
Mechanical  Energy,"  "  Proceedings  of  the  Royal  Society  of  Edin- 
burgh," April  19,  1852;  or  the  "Philosophical  Magazine,"  October, 
1852;  also  "Mathematical  and  Physical  Papers,"  vol.  i,  art.  59. 

43 


44  THOMSON 

heat  and  light.    The  subject  will  be  discussed  under  three 
heads: 

I.  The  secular  cooling  of  the  sun. 

II.  The  present  temperature  of  the  sun. 

III.  The  origin  and  total  amount  of  the  sun's  heat. 

I.  THE  SECULAR  COOLING  OF  THE  SUN. — How  much 
the  sun  is  actually  cooled  from  year  to  year,  if  at  all,  we 
have  no  means  of  ascertaining,  or  scarcely  even  of  esti- 
mating in  the  roughest  manner.  In  the  first  place  we  do 
not  know  that  he  is  losing  heat  at  all.  For  it  is  quite 
certain  that  some  heat  is  generated  in  his  atmosphere  by 
the  influx  of  meteoric  matter;  and  it  is  possible  that  the 
amount  of  heat  so  generated  from  year  to  year  is  sufficient 
to  compensate  the  loss  by  radiation.  It  is,  however,  also 
possible  that  the  sun  is  now  an  incandescent  liquid  mass, 
radiating  away  heat,  either  primitively  created  in  his  sub- 
stance, or,  what  seems  far  more  probable,  generated  by  the 
falling  in  of  meteors  in  past  times,  with  no  sensible  com- 
pensation by  a  continuance  of  meteoric  action. 

It  has  been  shown  3  that,  if  the  former  supposition  were 
true,  the  meteors  by  which  the  sun's  heat  would  have  been 
produced  during  the  last  2,000  or  3,000  years  must  have 
been  all  that  time  much  within  the  earth's  distance  from  the 
sun,  and  must  therefore  have  approached  the  central  body 
in  very  gradual  spirals;  because,  if  enough  of  matter  to 
produce  the  supposed  thermal  effect  fell  in  from  space  out- 
side the  earth's  orbit,  the  length  of  the  year  would  have 
been  very  sensibly  shortened  by  the  additions  to  the  sun's 
mass  which  must  have  been  made.  The  quantity  of  mat- 
ter annually  falling  in  must,  on  that  supposition,  have 
amounted  to  ^  of  the  earth's  mass,  or  to  irmjVuw  of  the 
sun's;  and  therefore  it  would  be  necessary  to  suppose 
the  "  zodiacal  light "  to  amount  to  at  least  7-gVir  of  the 
sun's  mass,  to  account  in  the  same  way  for  a  future  supply 
of  3,000  years'  sun-heat.  When  these  conclusions  were 

* "  On  the  Mechanical  Energies  of  the  Solar  System,"  "  Transac- 
tions of  the  Royal  Society  of  Edinburgh,"  April,  1854,  and  "  Philo- 
sophical Magazine,"  December,  1854. 


THE  AGE  OF  THE  SUN'S  HEAT         45 

first  published  it  was  pointed  out  that  "  disturbances  in  the 
motions  of  visible  planets  "  should  be  looked  for,  as  afford- 
ing us  means  for  estimating  the  possible  amount  of  matter 
in  the  zodiacal  light;  and  it  was  conjectured  that  it  could 
not  be  nearly  enough  to  give  a  supply  of  30,000  years'  heat 
at  the  present  rate.  These  anticipations  have  been  to  some 
extent  fulfilled  in  Le  Verrier's  great  researches  on  the 
motion  of  the  planet  Mercury,  which  have  recently  given 
evidence  of  a  sensible  influence  attributable  to  matter  cir- 
culating, as  a  great  number  of  small  planets,  within  his 
orbit  round  the  sun.  But  the  amount  of  matter  thus  indi- 
cated is  very  small;  and,  therefore,  if  the  meteoric  influx 
taking  place  at  present  is  enough  to  produce  any  appre- 
ciable portion  of  the  heat  radiated  away,  it  must  be  sup- 
posed to  come  from  matter  circulating  round  the  sun, 
within  very  short  distances  of  his  surface.  The  density  of 
this  meteoric  cloud  would  have  to  be  supposed  so  great 
that  comets  could  scarcely  have  escaped  as  comets  actu- 
ally have  escaped,  showing  no  discoverable  effects  of  re- 
sistance, after  passing  his  surface  within  a  distance  equal 
to  j-  of  his  radius.  All  things  considered,  there  seems 
little  probability,  in  the  hypothesis  that  solar  radiation 
is  at  present  compensated,  to  any  appreciable  degree,  by 
heat  generated  by  meteors  falling  in;  and,  as  it  can  be 
shown  that  no  chemical  theory  is  tenable,4  it  must  be  con- 
cluded as  most  probable  that  the  sun  is  at  present  merely 
an  incandescent  liquid  mass  cooling. 

How  much  he  cools  from  year  to  year  becomes  there- 
fore a  question  of  very  serious  import,  but  it  is  one  which 
we  are  at  present  quite  unable  to  answer.  It  is  true  we 
have  data  on  which  we  might  plausibly  found  a  probable 
estimate,  and  from  which  we  might  deduce,  with  at  first 
sight  seemingly  well-founded  confidence,  limits,  not  very 
wide,  within  which  the  present  true  rate  of  the  sun's  cool- 
ing must  lie.  For  we  know,  from  the  independent  but  con- 
cordant investigations  of  Herschel  and  Pouillet,  that  the 
sun  radiates  every  year  from  his  whole  surface  about 
4  "  Mechanical  Energies  of  the  Solar  System." 


46  THOMSON 

6  x  io30  (six  million  million  million  million  million)  times 
as  much  heat  as  is  sufficient  to  raise  the  temperature  of  one 
pound  of  water  by  i°  C.  We  also  have  excellent  reason 
for  believing  that  the  sun's  substance  is  very  much  like  the 
earth's.  Stokes's  principles  of  solar  and  stellar  chemistry 
have  been  for  many  years  explained  in  the  University  of 
Glasgow,  and  it  has  been  taught  as  a  first  result  that  sodium 
does  certainly  exist  in  the  sun's  atmosphere,  and  in  the 
atmospheres  of  many  of  the  stars,  but  that  it  is  not  discov- 
erable in  others.  The  recent  application  of  these  principles 
in  the  splendid  researches  of  Bunsen  and  Kirchhof  (who 
made  an  independent  discovery  of  Stokes's  theory)  has 
demonstrated  with  equal  certainty  that  there  are  iron  and 
manganese,  and  several  of  our  other  known  metals,  in  the 
sun.  The  specific  heat  of  each  of  these  substances  is  less 
than  the  specific  heat  of  water,  which  indeed  exceeds  that 
of  every  other  known  terrestrial  body,  solid  or  liquid.  It 
might,  therefore,  at  first  sight  seem  probable  that  the  mean 
specific  heat 5  of  the  sun's  whole  substance  is  less,  and  very 
certain  that  it  can  not  be  much  greater,  than  that  of  water. 
If  it  were  equal  to  the  specific  heat  of  water  we  should 
only  have  to  divide  the  preceding  number  (6  x  io30),  de- 
rived from  Herschel's  and  Pouillet's  observations,  by  the 
number  of  pounds  (4.3  x  io30)  in  the  sun's  mass,  to  find 
1.4°  C.  for  the  present  annual  rate  of  cooling.  It  might 
therefore  seem  probable  that  the  sun  cools  more,  and 
almost  certain  that  he  does  not  cool  less,  than  a  centigrade 
degree  and  four  tenths  annually.  But,  if  this  estimate  were 
well  founded,  it  would  be  equally  just  to  assume  that  the 

6  The  "  specific  heat  "  of  a  homogeneous  body  is  the  quantity  of  heat 
that  a  unit  of  its  substance  must  acquire  or  must  part  with,  to  rise  or 
to  fall  by  i°  in  temperature.  The  main  specific  heat  of  a  heterogeneous 
mass,  or  of  a  mass  of  homogeneous  substance,  under  different  pres- 
sures in  different  parts,  is  the  quantity  of  heat  which  the  whole  body 
takes  or  gives  in  rising  or  in  falling  i°  in  temperature,  divided  by  the 
number  of  units  in  its  mass.  The  expression,  "  mean  specific  heat "  of 
the  sun,  in  the  text,  signifies  the  total  amount  of  heat  actually  radiated 
away  from  the  sun,  divided  by  his  mass,  during  any  time  in  which  the 
average  temperature  of  his  mass  sinks  by  i°,  whatever  physical  or 
chemical  changes  any  part  of  his  substance  may  experience. 


THE  AGE  OF  THE  SUN'S  HEAT         47 

sun's  expansibility  6  with  heat  does  not  differ  greatly  from 
that  of  some  average  terrestrial  body.  If,  for  instance,  it 
were  the  same  as  that  of  solid  glass,  which  is  about  TirSinj 
on  bulk,  or  oihnnr  on  diameter,  per  i°  C.  (and  for  most 
terrestrial  liquids,  especially  at  high  temperatures,  the  ex- 
pansibility is  much  more),  and  if  the  specific  heat  were  the 
same  as  that  of  liquid  water,  there  would  be  in  860  years 
a  contraction  of  one  per  cent  on  the  sun's  diameter,  which 
Could  scarcely  have  escaped  detection  by  astronomical  ob- 
servation. There  is,  however,  a  far  stronger  reason  than 
this  for  believing  that  no  such  amount  of  contraction  could 
have  taken  place,  and  therefore  for  suspecting  that  the 
physical  circumstances  of  the  sun's  mass  render  the  con- 
dition of  the  substances  of  which  it  is  composed,  as  to  ex- 
pansibility and  specific  heat,  very  different  from  that  of  the 
same  substances  when  experimented  on  in  our  terrestrial 
laboratories.  Mutual  gravitation  between  the  different 
parts  of  the  sun's  contracting  mass  must  do  an  amount  of 
work,  which  can  not  be  calculated  with  certainty,  only  be- 
cause the  law  of  the  sun's  interior  density  is  not  known. 
The  amount  of  work  performed  on  a  contraction  of  one 
tenth  per  cent  of  the  diameter,  if  the  density  remained  uni- 
form throughout  the  interior,  would,  as  Helmholtz  showed, 
be  equal  to  20,000  times  the  mechanical  equivalent  of  the 
amount  of  heat  which  Pouillet  estimated  to  be  radiated 
from  the  sun  in  a  year.  But  in  reality  the  sun's  density 
must  increase  very  much  toward  his  centre,  and  probably 
in  varying  proportions,  as  the  temperature  becomes  lower 
and  the  whole  mass  contracts.  We  can  not,  therefore,  say 
whether  the  work  actually  done  by  mutual  gravitation  dur- 
ing a  contraction  of  one  tenth  per  cent  of  the  diameter 

'  The  "  expansibility  in  volume,"  or  the  "  cubical  expansibility,"  of 
a  body,  is  an  expression  technically  used  to  denote  the  proportion 
which  the  increase  or  diminution  of  its  bulk,  accompanying  a  rise  or 
fall  of  i°  in  its  temperature,  bears  to  its  whole  bulk  at  some  stated 
temperature.  The  expression,  "  the  sun's  expansibility,"  used  in  the 
text,  may  be  taken  as  signifying  the  ratio  which  the  actual  contrac- 
tion, during  a  lowering  of  his  mean  temperature  by  i°  C.,  bears  to 
his  present  volume. 


48  THOMSON 

would  be  more  or  less  than  the  equivalent  of  20,000  years' 
heat;  but  we  may  regard  it  as  most  probably  not  many 
times  more  or  less  than  this  amount.  Now,  it  is  in  the 
highest  degree  improbable  that  mechanical  energy  can  in 
any  case  increase  in  a  body  contracting  in  virtue  of  cool- 
ing. It  is  certain  that  it  really  does  diminish  very  notably 
in  every  case  hitherto  experimented  on.  It  must  be  sup- 
posed, therefore,  that  the  sun  always  radiates  away  in  heat 
something  more  than  the  Joule-equivalent  of  the  work 
done  on  his  contracting  mass,  by  mutual  gravitation  of 
its  parts.  Hence,  in  contracting  by  one  tenth  per  cent  in 
his  diameter,  or  three  tenths  per  cent  in  his  bulk,  the 
sun  must  give  out  something  either  more,  or  not  greatly 
less,  than  20,000  years'  heat;  and  thus,  even  without 
historical  evidence  as  to  the  constancy  of  his  diameter,  it 
seems  safe  to  conclude  that  no  such  contraction  as  that 
calculated  above  (one  per  cent  in  860  years)  can  have 
taken  place  in  reality.  It  seems,  on  the  contrary,  probable 
that,  at  the  present  rate  of  radiation,  a  contraction  of  one 
tenth  per  cent  in  the  sun's  diameter  could  not  take  place 
in  much  less  than  20,000  years,  and  scarcely  possible  that 
it  could  take  place  in  less  than  8,600  years.  If,  then,  the 
mean  specific  heat  of  the  sun's  mass,  in  its  actual  con- 
dition, is  not  more  than  ten  times  that  of  water,  the  ex- 
pansibility in  volume  must  be  less  than  ^Vir  Per  IOO°  C. 
(that  is  to  say,  less  than  •&  of  that  of  solid  glass),  which 
seems  improbable.  But  although  from  this  consideration 
we  are  led  to  regard  it  as  possible  that  the  sun's  specific 
heat  is  considerably  more  than  ten  times  that  of  water  (and, 
therefore,  that  his  mass  cools  considerably  less  than  100°  C. 
in  700  years,  a  conclusion  which,  indeed,  we  could  scarcely 
avoid  on  simply  geological  grounds),  the  physical  prin- 
ciples we  now  rest  on  fail  to  give  us  any  reason  for  sup- 
posing that  the  sun's  specific  heat  is  more  than  10,000 
times  that  of  water,  because  we  can  not  say  that  his  ex- 
pansibility in  volume  is  probably  more  than  ^J-g-  per  i°  C. 
And  there  is,  on  other  grounds,  very  strong  reason  for 
believing  that  the  specific  heat  is  really  much  less  than 


THE  AGE  OF  THE  SUN'S  HEAT         49 

10,000.  For  it  is  almost  certain  that  the  sun's  mean  tem- 
perature is  even  now  as  high  as  14,000°  C;  and  the  great- 
est quantity  of  heat  that  we  can  explain,  with  any  proba- 
bility, to  have  been  by  natural  causes  ever  acquired  by  the 
sun  (as  we  shall  see  in  the  third  part  of  this  article),  could 
not  have  raised  his  mass  at  any  time  to  this  temperature, 
unless  his  specific  heat  were  less  than  10,000  times  that  of 
water. 

We  may  therefore  consider  it  as  rendered  highly 
probable  that  the  sun's  specific  heat  is  more  than  ten  times, 
and  less  than  10,000  times,  that  of  liquid  water.  From 
this  it  would  follow  with  certainty  that  his  temperature 
sinks  1 00°  C.  in  some  time  from  700  years  to  700,000 
years. 

What,  then,  are  we  to  think  of  such  geological  esti- 
mates as  300,000,000  years  for  the  "  denudation  of  the 
Weald"?  Whether  is  it  more  probable  that  the  physical 
conditions  of  the  sun's  matter  differ  1,000  times  more  than 
dynamics  compel  us  to  suppose  they  differ  from  those  of 
matter  in  our  laboratories;  or  that  a  stormy  sea,  with  pos- 
sibly Channel  tides  of  extreme  violence,  should  encroach 
on  a  chalk  cliff  1,000  times  more  rapidly  than  Mr.  Dar- 
win's estimate  of  one  inch  per  century? 

II.  THE  PRESENT  TEMPERATURE  OF  THE  SUN. — At  his 
surface  the  sun's  temperature  can  not,  as  we  have  many 
reasons  for  believing,  be  incomparably  higher  than  tem- 
peratures attainable  artificially  in  our  terrestrial  labora- 
tories. 

Among  other  reasons  it  may  be  mentioned  that  the 
sun  radiates  out  heat  from  every  square  foot  of  his  surface 
at  only  about  7,000  horse  power.7  Coal,  burning  at  a  rate 
of  a  little  less  than  a  pound  per  two  seconds,  would  gen- 
erate the  same  amount;  and  it  is  estimated  (Rankine, 

7  One  horse  power  in  mechanics  is  a  technical  expression  (follow- 
ing Watt's  estimate)  used  to  denote  a  rate  of  working  in  which  energy 
is  involved  at  the  rate  of  33,000  foot  pounds  per  minute.  This,  according 
to  Joule's  determination  of  the  dynamical  value  of  heat,  would,  if  spent 
wholly  in  heat,  be  sufficient  to  raise  the  temperature  of  23!  pounds  of 
water  by  i°  C.  per  minute. 

4 


50  THOMSON 

"  Prime  Movers,"  p.  285,  edition  1852)  that,  in  the  fur- 
naces of  locomotive  engines,  coal  burns  at  from  one  pound 
in  thirty  seconds  to  one  pound  in  ninety  seconds  per 
square  foot  of  grate-bars.  Hence  heat  is  radiated  from  the 
sun  at  a  rate  not  more  than  from  fifteen  to  forty-five  times 
as  high  as  that  at  which  heat  is  generated  on  the  grate- 
bars  of  a  locomotive  furnace,  per  equal  areas. 

The  interior  temperature  of  the  sun  is  probably  far 
higher  than  that  at  his  surface,  because  direct  conduction 
can  play  no  sensible  part  in  the  transference  of  heat  be- 
tween the  inner  and  outer  portions  of  his  mass,  and  there 
must  be  an  approximate  convective  equilibrium  of  heat 
throughout  the  whole,  if  the  whole  is  fluid.  That  is  to  say, 
the  temperatures,  at  different  distances  from  the  centre, 
must  be  approximately  those  which  any  portion  of  the 
substance,  if  carried  from  the  centre  to  the  surface,  would 
acquire  by  expansion  without  loss  or  gain  of  heat. 

III.  THE  ORIGIN  AND  TOTAL  AMOUNT  OF  THE  SUN'S 
HEAT. — The  sun  being,  for  reasons  referred  to  above, 
assumed  to  be  an  incandescent  liquid  now  losing  heat,  the 
question  naturally  occurs,  How  did  this  heat  originate?  It 
is  certain  that  it  can  not  have  existed  in  the  sun  through 
an  infinity  of  past  time,  since,  as  long  as  it  has  so  existed, 
it  must  have  been  suffering  dissipation,  and  the  finiteness 
of  the  sun  precludes  the  supposition  of  an  infinite  primitive 
store  of  heat  in  his  body. 

The  sun  must,  therefore,  either  have  been  created  as 
an  active  source  of  heat  at  some  time  of  not  immeasurable 
antiquity,  by  an  overruling  decree;  or  the  heat  which  he 
has  already  radiated  away,  and  that  which  he  still  possesses, 
must  have  been  acquired  by  a  natural  process,  following 
permanently  established  laws.  Without  pronouncing  the 
former  supposition  to  be  essentially  incredible,  we  may 
safely  say  that  it  is  in  the  highest  degree  improbable,  if 
we  can  show  the  latter  to  be  not  contradictory  to  known 
physical  laws.  And  we  do  show  this  and  more,  by  merely 
pointing  to  certain  actions  going  on  before  us  at  present, 
which,  if  sufficiently  abundant  at  some  past  time,  must  haye 


THE  AGE  OF  THE  SUN'S  HEAT         51 

given  the  sun  heat  enough  to  account  for  all  we  know  of 
his  past  radiation  and  present  temperature. 

It  is  not  necessary  at  present  to  enter  at  length  on  de- 
tails regarding  the  meteoric  theory,  which  appears  to  have 
been  first  proposed  in  a  definite  form  by  Mayer,  and  after- 
ward independently  by  Waterston;  or  regarding  the  modi- 
fied hypothesis  of  meteoric  vortices,  which  the  writer  of 
the  present  article  showed  to  be  necessary,  in  order  that 
the  length  of  the  year,  as  known  for  the  last  2,000  years, 
may  not  have  been  sensibly  disturbed  by  the  accessions 
which  the  sun's  mass  must  have  had  during  that  period,  if 
the  heat  radiated  away  has  been  always  compensated  by 
heat  generated  by  meteoric  influx. 

For  reasons  mentioned  in  the  first  part  of  the  present 
article,  we  may  now  believe  that  all  theories  of  complete,  or 
nearly  complete,  contemporaneous  meteoric  compensation 
must  be  rejected;  but  we  may  still  hold  that  "meteoric 
action  ...  is  ...  not  only  proved  to  exist  as  a  cause 
of  solar  heat,  but  it  is  the  only  one  of  all  conceivable  causes 
which  we  know  to  exist  from  independent  evidence."  8 

The  form  of  meteoric  theory  which  now  seems  most 
probable,  and  which  was  first  discussed  on  true  thermo- 
dynamic  principles  by  Helmholtz,9  consists  in  supposing 
the  sun  and  his  heat  to  have  originated  in  a  coalition  of 
smaller  bodies,  falling  together  by  mutual  gravitation,  and 
generating,  as  they  must  do  according  to  the  great  law 
demonstrated  by  Joule,  an  exact  equivalent  of  heat  for  the 
motion  lost  in  collision. 

That  some  form  of  the  meteoric  theory  is  certainly  the 
true  and  complete  explanation  of  solar  heat  can  scarcely 
be  doubted,  when  the  following  reasons  are  considered: 

1.  No  other  natural  explanation,  except  by  chemical 
action,  can  be  conceived. 

2.  The  chemical  theory  is  quite  insufficient,  because  the 
most  energetic  chemical  action  we  know,  taking  place  be- 

"  Mechanical  Energies  of  the  Solar  System."    Note,  p.  351. 
*  Popular  lecture  delivered  on  February  7,  1854,  at  Konigsberg,  on 
the  occasion  of  the  Kant  commemoration. 


5  2  THOMSON 

tween  substances  amounting  to  the  whole  sun's  mass, 
would  only  generate  about  3,000  years'  heat.10 

3.  There  is  no  difficulty  in  accounting  for  20,000,000 
years'  heat  by  the  meteoric  theory. 

It  would  extend  this  article  to  too  great  a  length,  and 
would  require  something  of  mathematical  calculation,  to 
explain  fully  the  principles  on  which  this  last  estimate  is 
founded.  It  is  enough  to  say  that  bodies,  all  much  smaller 
than  the  sun,  falling  together  from  a  state  of  relative  rest, 
at  mutual  distances  all  large  in  comparison  with  their  di- 
ameters, and  forming  a  globe  of  uniform  density  equal  in 
mass  and  diameter  to  the  sun,  would  generate  an  amount 
of  heat  which,  accurately  calculated  according  to  Joule's 
principles  and  experimental  results,  is  found  to  be  just 
20,000,000  times  Pouillet's  estimate  of  the  annual  amount 
of  solar  radiation.  The  sun's  density  must,  in  all  proba- 
bility, increase  very  much  toward  his  centre,  and  therefore 
a  considerably  greater  amount  of  heat  than  that  must  be 
supposed  to  have  been  generated  if  his  whole  mass  was 
formed  by  the  coalition  of  comparatively  small  bodies.  On 
the  other  hand,  we  do  not  know  how  much  heat  may  have 
been  dissipated  by  resistance  and  minor  impacts  before  the 
final  conglomeration;  but  there  is  reason  to  believe  that 
even  the  most  rapid  conglomeration  that  we  can  conceive 
to  have  probably  taken  place  could  only  leave  the  finished 
globe  with  about  half  the  entire  heat  due  to  the  amount  of 
potential  energy  of  mutual  gravitation  exhausted.  We 
may,  therefore,  accept,  as  a  lowest  estimate  for  the  sun's 
initial  heat,  10,000,000  times  a  year's  supply  at  the  present 
rate,  but  50,000,000  or  100,000,000  as  possible,  in  conse- 
quence of  the  sun's  greater  density  in  his  central  parts. 

The  considerations  adduced  above,  in  this  paper,  re- 
garding the  sun's  possible  specific  heat,  rate  of  cooling,  and 
superficial  temperature,  render  it  probable  that  he  must 
have  been  very  sensibly  warmer  1,000,000  years  ago  than 
now;  and,  consequently,  if  he  has  existed  as  a  luminary  for 
10,000,000  or  20,000,000  years,  he  must  have  radiated  away 

10 "  Mechanical  Energies  of  the  Solar  System."     Note,  p.  351. 


THE  AGE  OF  THE  SUN'S  HEAT         53 

considerably  more  than  the  corresponding  number  of  times 
the  present  yearly  amount  of  loss. 

It  seems,  therefore,  on  the  whole  most  probable  that  the 
sun  has  not  illuminated  the  earth  for  100,000,000  years,  and 
almost  certain  that  he  has  not  done  so  for  500,000,000 
years.  As  for  the  future,  we  may  say,  with  equal  certainty, 
that  inhabitants  of  the  earth  can  not  continue  to  enjoy  the 
light  and  heat  essential  to  their  life  for  many  million  years 
longer  unless  sources  now  unknown  to  us  are  prepared 
in  the  great  storehouse  of  creation. 


THE  PAST  AND  FUTURE  OF 
OUR  EARTH 

AND 

A  NEW  THEORY  OF  LIFE  IN 
OTHER  WORLDS 

BY 

RICHARD   ANTHONY   PROCTOR 
(Born  1837  ;  died  1888) 


THE  PAST  AND  FUTURE  OF  OUR  EARTH1 

"  Ut  his  exordia  primis 

Omnia,  et  ipse  tener  Mundi  concreverit  orbis. 
Turn  durare  solum,  et  discludere  Nerea  ponto 
Coeperit,  et  rerum  paullatim  sumere  formas." 

VIRGIL. 

THE  subject  with  which  I  am  about  to  deal 2  is  asso- 
ciated by  many  with  questions  of  religion.  Let  me 
premise,  however,  that  I  do  not  thus  view  it  myself. 
It  seems  to  me  impossible  to  obtain  from  science  any  clear 
ideas  respecting  the  ways  or  nature  of  the  Deity,  or  even 
respecting  the  reality  of  an  Almighty  personal  God.  Sci- 
ence deals  with  the  finite,  though  it  may  carry  our  thoughts 
to  the  infinite.  Infinity  of  space  and  of  matter  occupying 
space,  of  time  and  of  the  processes  with  which  time  is  occu- 
pied, and  infinity  of  energy  as  necessarilv  implied  by  the 
infinities  of  matter  and  of  the  operations  affecting  matter — 
these  infinities  science  brings  clearly  before  us.  For  sci- 
ence directs  our  thoughts  to  the  finites  to  which  these  in- 
finites correspond.  It  shows  us  that  there  can  be  no  con- 
ceivable limits  to  space  or  time,  and  though  finiteness  of 
matter  or  of  operation  may  be  conceivable,  there  is  mani- 
fest incongruity  in  assuming  an  infinite  disproportion  be- 
tween unoccupied  and  occupied  space,  or  between  void 

1  From  "  Our  Place  among  Infinities,"  D.  Appleton  and  Company. 

*  This  essay  presents  the  substance  of  a  lecture  delivered  in  New 
York  on  April  3,  1874,  being  the  first  of  a  subsidiary  series  in  which, 
of  set  purpose  (and  in  accordance  with  the  request  of  several  esteemed 
friends),  I  dealt  less  with  the  direct  teachings  of  astronomy,  which 
had  occupied  me  in  a  former  series,  than  with  ideas  suggested  by 
astronomical  facts,  and  more  particularly  by  the  discoveries  made  dur- 
ing the  last  quarter  of  a  century. 

57 


58  PROCTOR 

time  and  time  occupied  with  the  occurrence  of  events  of 
what  sort  soever.  So  that  the  teachings  of  science  bring 
us  into  the  presence  of  the  unquestionable  infinities  of 
time  and  of  space,  and  the  presumable  infinities  of  mat- 
ter and  of  operation — hence,  therefore,  into  the  presence 
of  infinity  of  energy.  But  science  teaches  us  nothing  about 
these  infinities,  as  such.  They  remain  none  the  less  incon- 
ceivable, however  clearly  we  may  be  taught  to  recognise 
their  reality.  Moreover,  these  infinites,  including  the  in- 
finity of  energy,  are  material  infinities.  Science  tells  us 
nothing  of  the  infinite  attributes  of  an  Almighty  Being;  it 
presents  to  us  no  personal  infinites,  whether  of  Power, 
Beneficence,  or  Wisdom.  Science  may  suggest  some  ideas 
on  these  points,  though  we  perceive  daily  more  and  more 
clearly  that  it  is  unsafe  to  accept  as  her  teaching  ideas 
which  commonly  derive  their  colouring  from  our  own  pre- 
possessions. And  assuredly,  as  respects  actual  facts,  Sci- 
ence in  so  far  as  she  presents  personal  infinity  to  us  at  all, 
presents  it  as  an  inconceivable,  like  those  other  inconceiv- 
able infinities,  with  the  finites  corresponding  to  which  her 
operations  are  alone  directly  concerned.  To  speak  in  plain 
terms — so  far  as  science  is  concerned,  the  idea  of  a  per- 
sonal God  is  inconceivable,3  as  are  all  the  attributes  which 
religion  recognises  in  such  a  Being.  On  the  other  hand, 
it  should  be  admitted  as  distinctly,  that  Science  no  more 
disproves  the  existence  of  infinite  personal  power  or  wis- 
dom than  she  disproves  the  existence  of  infinite  material 

*  I  mean  these  words  to  be  understood  literally.  To  the  man  of 
science,  observing  the  operation  of  second  causes  in  every  process  with 
which  his  researches  deal,  and  finding  no  limit  to  the  operation  of 
such  causes  however  far  back  he  may  trace  the  chain  of  causation,  the 
idea  of  a  first  cause  is  as  inconceivable  in  its  relation  to  observed  sci- 
entific facts  as  is  the  idea  of  infinite  space  in  its  relation  to  the  finite 
space  to  which  the  observations  of  science  extend.  Yet  infinite  space 
must  be  admitted;  nor  do  I  see  how  even  that  man  of  science  who 
would  limit  his  thoughts  most  rigidly  to  facts,  can  admit  that  all  things 
are  of  which  he  thinks,  without  having  impressed  upon  him  the  feeling 
that,  in  some  way  he  can  not  understand,  these  things  represent  the 
operation  of  Infinite  Purpose.  Assuredly  we  do  not  avoid  the  incon- 
ceivable by  assuming  as  at  least  possible  that  matter  exists  only  as  it 
affects  our  perceptions. 


THE  PAST  AND  FUTURE  OF  OUR  EARTH     59 

energy  (which  on  the  contrary  must  be  regarded  as  prob- 
able) or  the  existence  of  infinite  space  or  time  (which  must 
be  regarded  as  certain). 

So  much  premised,  we  may  proceed  to  inquire  into  the 
probable  past  and  future  of  our  earth,  as  calmly  as  we 
should  inquire  into  the  probable  past  and  future  of  a 
pebble,  a  weed,  or  an  insect,  of  a  rock,  a  tree,  or  an  animal, 
of  a  continent,  or  of  a  type — whether  of  vegetable  or  of 
animal  life.  The  beginning  of  all  things  is  not  to  be 
reached,  not  appreciably  to  be  even  approached,  by  a  few 
steps  backward  in  imagination,  nor  the  end  of  all  things  by 
a  few  steps  forward.  Such  a  thought  is  as  unfounded  as 
was  the  fear  of  men  in  old  times  that  by  travelling  too  far 
in  any  direction  they  might  pass  over  the  earth's  edge  and 
be  plunged  into  the  abyss  beyond,  as  unreasonable  as  was 
the  hope  that  by  increase  of  telescopic  range  astronomers 
could  approach  the  imagined  "  heavens  above  the  crys- 
talline." 

In  considering  the  probable  past  history  of  the  earth, 
we  are  necessarily  led  to  inquire  into  the  origin  of  the 
solar  system.  I  have  already  sketched  two  theories  of  the 
system,  and  described  the  general  facts  on  which  both 
theories  are  based.  The  various  planets  circle  in  one  direc- 
tion around  the  sun,  the  sun  rotating  in  the  same  direction, 
the  satellite  families  (with  one  noteworthy  but  by  no  means 
inexplicable  exception)  travelling  round  their  primaries  in 
the  same  direction,  and  all  the  planets  whose  rotation  has 
been  determined  still  preserving  the  same  direction  of  cir- 
culation (so  to  speak).  These  relations  seem  to  point,  in 
a  manner  there  is  no  mistaking,  to  a  process  of  evolution 
by  which  those  various  parts  of  the  solar  system  which 
now  form  discrete  masses  were  developed  from  a  former 
condition  characterized  by  a  certain  unity  as  respects  the 
manner  of  its  circulation.  One  theory  of  this  process  of 
evolution,  Laplace's,  implies  the  contraction  of  the  solar 
system  from  a  great  rotating  nebulous  mass;  according  to 
the  other  theory,  the  solar  system,  instead  of  contracting  to 
its  present  condition,  was  formed  by  a  process  of  accre- 


60  PROCTOR 

tion,  due  to  the  indrawing  of  great  flights  of  meteoric  and 
cometic  matter. 

I  need  not  here  enter  at  length,  for  I  have  already  done 
so  elsewhere,  into  the  astronomical  evidence  in  favour  of 
either  theory;  but  it  will  be  well  to  present  briefly  some 
of  the  more  striking  facts. 

Among  the  various  forms  of  nebulae  (or  star-cloudlets) 
revealed  by  the  telescope,  we  find  many  which  seem  to 
accord  with  our  ideas  as  to  some  of  the  stages  through 
which  our  solar  system  must  have  passed  in  changing  from 
the  nebulous  condition  to  its  present  form.  The  irregular 
nebulae — such,  for  instance,  as  that  wonderful  nebula  in  the 
Sword  of  Orion — show  by  their  enormous  extension  the 
existence  of  sufficient  quantities  of  gaseous  matter  to  form 
systems  as  large  and  as  massive  as  our  own,  or  even  far 
vaster.  We  know  from  the  teachings  of  the  spectroscope 
that  these  irregular  nebulae  do  really  consist  of  glowing 
gas  (as  Sir  W.  Herschel  long  since  surmised),  hydrogen 
and  nitrogen  being  presumably  present,  though  the  spec- 
trum of  neither  gas  appears  in  its  complete  form  (one  line 
only  of  each  spectrum  being  shown,  instead  of  the  sets  of 
lines  usually  given  by  these  gases).  An  American  physicist 
has  suggested  that  hydrogen  and  nitrogen  exist  in  the  gase- 
ous nebulae  in  an  elementary  condition,  these  gases  really  be- 
ing compound,  and  he  suggests  further  that  all  our  so-called 
elements  may  have  been  derived  from  those  elementary 
forms  of  hydrogen  and  nitrogen.  In  the  absence  of  any 
evidence  from  observation  or  experiment,  these  ideas  must 
be  regarded  as  merely  speculative;  and  I  think  that  we 
arrive  here  at  a  point  where  speculation  helps  us  as  little 
as  it  does  in  attempting  to  trace  the  evolution  of  living 
creatures  across  the  gap  which  separates  the  earliest  forms 
of  life  from  the  beginning  itself  of  life  upon  the  earth. 
Since  we  can  not  hope  to  determine  the  real  beginning  of 
the  earth's  history,  we  need  not  at  present  attempt  to  pass 
back  beyond  the  earliest  stage  of  which  we  have  any  clear 
information. 

Passing  from  the  irregular  nebulae,  in  which  we  see 


THE   XEBULA   IN  ORIOX. 
Photogravure  from  a  photograph  taken  at  the  Lick  Observatory 


n,  due  to  the  c  flights  of  meteoric  and 
cometic  matter. 

I  need  not  ^h,  for  I  have  already  done 

so  elsewher"  d  evidence  in  favour  of 

either  t  veil  to  present  briefly  some 
of  the  n. 

Am  .ebulae  (or  star-cloudlets) 

revt  we  find  many  \vhich  seem  to 

eas  as  to  some  of  the  stages  through 

n  changing  from 

The  irregular 

nebula  in  the 

sion  the 

lorm 

far 

.•^flrarwbma  w'T«m- 


and  nitrogen  being  presumably  present,  though  the  spec- 

trum of  neither  gas  appears  in  its  complete  form  (one  line 

cctrum  being  shown,  instead  of  the  sets  of 

An  American  physicist 

r  in  the  gase- 

be- 

!!ed 

any 
must 

that  we 
arrh  ;ttje 

as  it  do<  ,f  Hying 

creature^  earliest  forms 

of  life  from  ,>on  the  earth. 

Since  we  ca  mine  the  real  beginning  of 

the  earth's  histc  not  at  present  attempt  to  pass 

beyond  the  eu;              ge  of  which  we  have  any  clear 

mation. 

Passing  from  the  irregular  nebulas,  in  which  we  see 


THE  PAST  AND  FUTURE  OF  OUR  EARTH    6l 

chaotic  masses  of  gaseous  matter  occupying  millions  of 
millions  of  cubic  miles  and  scattered  as  wildly  through 
space  as  clouds  are  scattered  in  a  storm-swept  air,  we  come 
to  various  orders  of  nebulae  in  which  we  seem  to  find  clear 
evidence  of  a  process  of  evolution.  We  see  first  the  traces 
of  a  central  aggregation.  This  aggregation  becomes  more 
and  more  clearly  defined,  until  there  is  no  possibility  of  mis- 
taking its  nature  as  a  centre  having  power  (by  virtue  of  the 
quantity  of  matter  contained  in  it)  to  influence  the  motions 
of  the  matter  belonging  to  the  rest  of  the  nebula.  Then, 
still  passing  be  it  remembered  from  nebula  to  nebula,  and 
only  inferring,  not  actually  witnessing,  the  changes  de- 
scribed— we  see  a  subordinate  aggregation,  wherein,  after 
a  while,  the  greater  portion  of  the  mass  of  the  nebula  out- 
side the  central  aggregation  becomes  gathered,  even  as 
Jupiter  contains  the  greater  portion  of  the  mass  of  the  solar 
system  outside  the  central  sun.4  Next  we  see  a  second  sub- 
ordinate aggregation,  inferior  to  the  first,  but  comprising, 
if  we  judge  from  its  appearance,  by  far  the  greater  portion 
of  what  remained  after  the  first  aggregation  had  been 
formed — even  as  Saturn's  mass  far  exceeds  the  combined 
mass  of  all  the  planets  less  than  himself,  and  so  comprises 
far  the  greater  portion  of  the  solar  system  after  account  has 
been  taken  of  Jupiter  and  the  sun.5  And  we  may  infer  that 
the  other  parts  of  nebulae  contain  smaller  aggregations  not 
perceptible  to  us,  out  of  which  the  smaller  planets  of  the 
developing  system  are  hereafter  to  be  formed. 

Side  views  of  some  of  these  nebulae  indicate  a  flatness 
of  figure  agreeing  well  with  the  general  tendency  of  the 
members  of  the  solar  system  toward  the  medial  plane  of 
that  system.  For  the  solar  system  may  be  described  as 
flat,  and  if  the  nebulae  I  have  been  dealing  with  (the  spiral 
nebulae  with  aggregations)  were  globular  we  could  not  rec- 
ognise in  them  the  true  analogues  of  our  solar  system  in  the 

4  The  mass  of  Jupiter  exceeds,  in  the  proportion  of  five  to  two,  the 
combined  mass  of  all  the  remaining  planets. 

8  The  mass  of  Saturn  exceeds,  in  the  proportion  of  nearly  three  to 
one,  the  combined  mass  of  all  the  planets  smaller  than  himself. 


62  PROCTOR 

earlier  stages  of  its  history.  But  the  telescope  reveals 
nebulae  manifestly  corresponding  in  appearance  to  the 
great  whirlpool  nebula  of  Lord  Rosse,  as  it  would  appear 
if  it  is  a  somewhat  flattened  spiral  and  could  be  viewed 
nearly  edgewise. 

And  here  I  may  pause  to  note  that,  although,  in  thus 
inferring  progressive  changes  where  in  reality  we  have  but 
various  forms  of  nebulae,  I  have  been  adopting  an  assump- 
tion and  one  which  no  one  can  hope  either  to  verify  or  to 
disprove,  yet  it  must  be  remembered  that  these  nebulae  by 
their  very  figure  indicate  that  they  are  not  at  rest.  If  they 
consist  of  matter  possessing  the  attribute  of  gravitation — 
and  it  would  be  infinitely  more  daring  to  assert  that  they 
do  not  than  that  they  do — then  they  must  be  undergoing 
processes  of  change.  Nor  can  we  conceive  that  discrete 
gaseous  masses  in  whorls  spirally  arranged  around  a  great 
central  aggregation  (taking  one  of  the  earlier  stages)  could 
otherwise  change  than  by  aggregating  toward  their  centre, 
unless  we  admit  motions  of  revolution  (in  orbits  more  or 
less  eccentric)  the  continuance  of  which  would  necessarily 
lead,  through  collisions,  to  the  rapid  growth  of  the  central 
aggregation,  and  to  the  formation  and  slower  growth  of 
subordinate  gatherings. 

I  have  shown  elsewhere  how  the  formation  of  our  solar 
system,  in  the  manner  supposed,  would  explain  what  La- 
place admitted  that  he  could  not  explain  by  his  theory — 
the  peculiar  arrangement  of  the  masses  forming  the  solar 
system.  The  laws  of  dynamics  tell  us  that  no  matter  what 
the  original  configuration  or  motion  of  the  masses,  prob- 
ably gaseous,  forming  the  nebula,  the  motions  of  these 
masses  would  have  greater  and  greater  velocity  the  nearer 
the  masses  were  to  the  central  aggregation,  each  distance 
indicating  certain  limits  between  which  the  velocities  must 
inevitably  lie.  For  example,  in  our  solar  system,  supposing 
the  central  sun  had  already  attained  very  nearly  his  full 
growth  as  respects  quantity  of  matter,  then  the  velocity  of 
any  mass  whatever  belonging  to  the  system  would  at 
Jupiter's  distance  be  less  than  twelve  miles  per  second, 


THE  PAST  AND  FUTURE  OF  OUR  EARTH     63 

whereas  at  the  distance  of  the  earth,  the  largest  planet 
travelling  inside  the  orbit  of  Jupiter,  the  limit  of  the  velocity 
would  be  more  than  twice  as  great.  Hence  we  can  see  with 
what  comparative  difficulty  an  aggregation  would  form  close 
to  the  central  one,  and  how  the  first  subordinate  aggregation 
would  lie  at  a  distance  where  the  quantity  of  matter  was 
still  great  but  the  average  velocity  of  motion  not  too  great. 
Such  an  aggregation  once  formed,  the  next  important 
aggregation  would  necessarily  lie  far  outside,  for  within 
the  first  there  would  now  be  two  disturbing  influences  pre- 
venting the  rapid  growth  of  these  aggregations.  The  third 
and  fourth  would  be  outside  the  second.  Between  the 
first  aggregation  and  the  sun  only  small  planets,  like  the 
earth  and  Venus,  Mars,  Mercury,  and  the  asteroids,  could 
form;  and  we  should  expect  to  find  that  the  largest  of  the 
four  small  planets  would  be  in  the  middle  of  the  space  be- 
longing to  the  family  (as  Venus  and  the  earth  are  actually 
placed),  while  the  much  smaller  planets  Mercury  and  Mars 
travel  next  on  either  side,  one  close  to  the  sun  and  the  other 
next  to  Jupiter,  the  asteroids  indicating  the  region  where 
the  combined  disturbing  influences  of  Jupiter  and  the  sun 
prevented  any  single  planet  from  being  developed. 

But  I  should  require  much  more  time  than  is  now  at 
my  command  to  present  adequately  the  reasoning  on  which 
the  theory  of  accretion  is  based.  And  we  are  not  concerned 
here  to  inquire  whether  this  theory,  or  Laplace's  theory  of 
contraction,  or  (which  I  hold  to  be  altogether  more  prob- 
able than  either)  a  theory  involving  combined  processes 
of  accretion  and  contraction,  be  the  true  hypothesis  of  the' 
evolution  of  the  solar  system.  Let  it  suffice  that  we  recog- 
nise as  one  of  the  earliest  stages  of  our  earth's  history  her 
condition  as  a  rotating  mass  of  glowing  vapour,  capturing 
then  as  now,  but  far  more  actively  then  than  now,  masses 
of  matter  which  approached  near  enough,  and  growing  by 
these  continual  indraughts  from  without.  From  the  very 
beginning,  as  it  would  seem,  the  earth  grew  in  this  way. 
This  firm  earth  on  which  we  live  represents  an  aggregation 
of  matter  not  from  one  portion  of  space,  but  from  all  space. 


64  PROCTOR 

All  that  is  upon  and  within  the  earth,  all  vegetable  forms 
and  all  animal  forms,  our  bodies,  our  brains,  are  formed  of 
materials  which  have  been  drawn  in  from  those  depths  of 
space  surrounding  us  on  all  sides.  This  hand  that  I  am 
now  raising  contains  particles  which  have  travelled  hither 
from  regions  far  away  amid  the  northern  and  southern  con- 
stellations, particles  drawn  in  toward  the  earth  by  processes 
continuing  millions  of  millions  of  ages,  until  after  multi- 
tudinous changes  the  chapter  of  accidents  has  so  combined 
them,  and  so  distributed  them  in  plants  and  animals,  that 
after  coming  to  form  portions  of  my  food  they  are  here 
present  before  you.  Passing  from  the  mere  illustration  of 
the  thought,  is  not  the  thought  itself  striking  and  sugges- 
tive, that  not  only  the  earth  on  which  we  move,  but  every- 
thing we  see  or  touch,  and  every  particle  in  body  and  brain, 
has  sped  during  countless  ages  through  the  immensity  of 
space? 

The  great  mass  of  glowing  gas  which  formed  our  earth 
in  the  earliest  stage  of  its  history  wras  undergoing  two  note- 
worthy processes — first,  the  process  of  cooling  by  which 
the  mass  was  eventually  to  become  at  least  partially  solid, 
and  secondly  a  process  of  growth  due  to  the  gathering  in 
of  meteoric  and  cometic  matter.  As  respects  the  latter 
process,  which  will  not  hereafter  occupy  our  attention,  I 
must  remark  that  many  astronomers  appear  to  me  to  give 
far  less  consideration  to  the  inferences  certainly  deducible 
from  recent  discoveries  than  the  importance  of  these  dis- 
coveries would  fairly  warrant.  It  is  now  absolutely  certain 
that  hour  by  hour,  day  by  day,  and  year  by  year,  the  earth 
is  gathering  in  matter  from  without.  On  the  most  mod- 
erate assumption  as  to  the  average  weight  of  meteors  and 
shooting  stars,  the  earth  must  increase  each  year  in  mass 
by  many  thousands  of  tons.  And  when  we  consider  the 
enormous,  one  may  almost  say  the  awful,  time-intervals 
which  have  elapsed  since  the  earth  was  in  a  gaseous  condi- 
tion, we  can  not  but  perceive  that  the  process  of  accretion 
now  going  on  indicates  the  existence  of  only  the  merest 
residue  of  matter  (ungathered)  compared  with  that  which 


THE  PAST  AND  FUTURE  OF  OUR  EARTH    65 

at  the  beginning  of  those  time-intervals  was  freely  moving 
around  the  central  aggregation.  The  process  of  accretion 
which  now  does  not  sensibly  increase  the  earth's  mass  was 
then  a  process  of  actual  growth.  Jupiter  and  Saturn  might 
then  no  longer  be  gathering  in  matter  appreciably  increasing 
their  mass,  although  the  quantity  of  matter  gathered  in  by 
them  must  have  been  far  larger  than  all  that  the  other  form- 
ing earth  could  gather  in  equal  times.  For  those  planets 
were  then  as  now  so  massive  that  any  possible  increment 
from  without  was  as  nothing  compared  with  the  mass  they 
had  already  attained.  We  have  to  throw  back  into  yet 
more  awful  time-depths  the  birth  and  growth  of  those  giant 
orbs.  And  even  those  depths  of  time  are  as  nothing  com- 
pared with  the  intervals  which  have  elapsed  since  the  sun 
himself  began  to  be.  Yet  it  is  with  time-intervals  measur- 
able by  hundreds  of  millions  of  years  that  we  have  to  deal 
in  considering  only  our  earth's  history — nay,  two  or  three 
hundred  millions  of  years  only  carry  us  back  to  a  period 
when  the  earth  was  in  a  stage  of  development  long  sequent 
to  the  gaseous  condition  we  are  now  considering.  That  the 
supply  of  meteoric  and  cometic  matter  not  gathered  in 
was  then  enormously  greater  than  that  which  still  exists 
within  the  solar  domain,  appears  to  me  not  a  mere  fanciful 
speculation,  nor  even  a  theoretical  consideration,  but  as 
nearly  a  certainty  as  anything  not  admitting  of  mathe- 
matical demonstration  can  possibly  be.  That  the  rate  of 
ingathering  at  that  time  enormously  exceeded  the  present 
rate  may  be  regarded  as  certain.  That  the  increase  result- 
ing from  such  ingathering  during  the  hundreds  of  millions 
of  years  that  it  has  been  in  operation  since  the  period  when 
the  earth  first  existed  as  a  gaseous  mass,  must  have  resulted 
in  adding  a  quantity  of  matter  forming  no  inconsiderable 
aliquot  part  of  the  earth's  present  mass,  seems  to  me  a 
reasonable  inference,  although  it  is  certain  that  the  present 
rate  of  growth  continued  even  for  hundreds  of  millions  of 
years  would  not  appreciably  affect  the  earth's  mass.6  And 

c  It  is,  perhaps,  hardly  necessary  to  explain  that  I  refer  here  not  to 
absolute  but  to  relative  increase.    The  absolute  increase  of  mass  would 
5 


66  PROCTOR 

it  is  a  thought  worthy  of  consideration,  in  selecting  be- 
tween Laplace's  theory  of  contraction  and  the  theory  of 
accretion,  that  accretion  being  a  process  necessarily  ex- 
haustive, we  are  able  to  trace  it  back  through  stages  of 
gradually  increasing  activity  without  limit  until  we  reach 
that  stage  when  the  whole  of  the  matter  now  forming  our 
solar  system  was  as  yet  unformed.  Contraction  may 
alternate  with  expansion,  according  to  the  changing  con- 
dition of  a  forming  system;  but  accretion  is  a  process 
which  can  only  act  in  one  direction;  and  as  accretion  is  cer- 
tainly going  on  now,  however  slowly,  we  have  but  to  trace 
back  the  process  to  be  led  inevitably,  in  my  judgment,  to 
regard  our  system  as  having  its  origin  in  processes  of  accre- 
tion— though  it  seems  equally  clear  that  each  individual 
orb  of  the  system,  if  not  each  subordinate  scheme  within 
.it,  has  also  undergone  a  process  of  contraction  from  a  for- 
mer nebulous  condition. 

In  this  early  gaseous  stage  our  earth  was  preparing,  as 
it  were,  to  become  a  sun.  As  yet  her  gaseous  globe  proba- 
bly extended  beyond  the  smaller  aggregation  out  of  which 
the  moon  was  one  day  to  be  formed.  This  may  be  inferred, 
I  think,  from  the  law  of  the  moon's  rotation.  It  is  true 
that  a  moon  independently  created,  and  started  on  the 
moon's  present  course,  with  a  rotation-period  nearly  equal- 
ling its  period  of  revolution,  would  gradually  have  acquired 
a  rotation-period  exactly  equalling  the  mean  period  of 
revolution.  But  there  is  no  reason  in  Nature  why  there 
should  have  been  any  such  near  approach;  whereas,  if  we 
suppose  the  moon's  gaseous  globe  to  have  been  originally 
entangled  within  the  outskirts  of  the  earth's,  we  see  that 
the  peculiar  relation  in  question  would  have  prevailed  from 
the  beginning  of  the  moon's  existence  as  a  separate  body. 
The  laws  of  dynamics  show  us,  moreover,  that  although 
the  conditions  under  which  the  moon  moved  and  rotated 
must  have  undergone  considerable  changes  since  her  first 
formation,  yet  that  since  those  changes  took  place  very 

amount  to  many  millions  of  tons,  but  the  earth  would  not  be  increased 
by  the  billionth  part  of  her  present  mass. 


THE  PAST  AND  FUTURE  OF  OUR  EARTH     67 

slowly,  the  rotation  of  the  moon  would  be  gradually  modi- 
fied, pari  passu,  so  that  the  peculiar  relation  between  the 
moon's  rotation  and  revolution  would  continue  unim- 
paired.7 

In  her  next  stage,  our  earth  is  presented  to  us  as  a 
sun.  It  may  be  that  at  that  time  the  moon  was  the  abode 
of  life,  our  earth  affording  the  supplies  of  light  and  heat 
necessary  for  the  wants  of  creatures  living  on  the  moon. 
But  whether  this  were  so  or  not,  it  may  be  safely  assumed 
that  when  the  earth's  contracting  gaseous  globe  first  began 
to  have  liquid  or  solid  matter  in  its  constitution,  the  earth 
must  have  been  a  sun  so  far  as  the  emission  of  heat  and 
light  were  concerned.  I  must  warn  you,  however,  against 
an  undue  regard  for  analogy  which  has  led  some  astrono- 
mers to  say  that  all  the  members  of  the  solar  system  have 
passed  or  will  pass  through  exactly  similar  stages.  That 
our  earth  once  gave  out  light  and  heat,  as  the  sun  does 
now,  may  be  admitted  as  probable;  and  we  may  believe 
that  later  the  earth  presented  the  characteristics  which  we 
now  recognise  in  Jupiter;  while  hereafter  it  may  pass 
through  a  stage  comparable  with  that  through  which  our 
moon  is  now  passing.  But  we  must  remember  that -the 
original  quantity  of  matter  in  any  orb  passing  through  such 
stages  must  very  importantly  modify  the  actual  condition 
of  the  orb  in  each  of  those  stages,  as  well,  of  course,  as  the 
duration  of  each  stage;  and  it  may  even  be  that  no  two 
orbs  in  the  universe  were  ever  in  the  same  or  very  nearly 
the  same  condition,  and  that  no  change  undergone  by  one 
has  corresponded  closely  with  any  change  undergone  by 
another. 

We  know  so  little  respecting  the  sun's  actual  condition, 
that  even  if  we  could  be  assured  that  in  any  past  stages 
of  her  history  the  earth  was  nearly  in  the  same  state,  we 

7  On  the  theory  of  evolution  some  such  view  of  the  origin  of  the 
moon's  rotation  must  be  adopted,  unless  the  matter  be  regarded  as 
the  result  of  a  strange  chance.  If  we  believe,  on  the  contrary,  that  the 
arrangement  was  specially  ordained  by  the  Creator,  we  are  left  to  won- 
der what  useful  purpose  a  relation  so  peculiar  and  so  artificial  can  have 
been  intended  to  subserve. 


68  PROCTOR 

should  nevertheless  remain  in  almost  complete  ignorance 
as  to  the  processes  to  which  the  earth's  orb  was  at  that  time 
subject.  In  particular  we  have  no  means  of  forming  an 
opinion  as  to  the  manner  in  which  the  elementary  con- 
stituents of  the  earth's  globe  were  situated  when  she  was 
in  the  sunlike  stage.  We  may  adopt  some  general  theory 
of  the  sun's  present  condition;  for  example,  we  may 
accept  the  ingenious  reasoning  by  which  Professor  Charles 
A.  Young  has  supported  his  theory  that  the  sun  is  a 
gigantic  bubble;8  but  we  should  be  far  from  having  any 
exact  idea  of  the  processes  actually  taking  place  within 
the  solar  globe,  even  if  we  were  absolutely  certain  that  that 
or  some  other  general  theory  were  the  true  one. 

Assuming  that  our  earth,  when  in  the  sunlike  stage,  was 
a  gaseous  mass  within  a  liquid  non-permanent  shell,  we 
can  see  that  as  the  process  of  cooling  went  on  the  showers 
forming  the  shell  would  attain  a  greater  and  greater  depth, 
the  shell  thus  becoming  thicker,  the  space  within  the  shell 

8 "  The  eruptions  which  are  all  the  time  "  (Anglice,  "  always  ") 
M  occurring  on  the  sun's  surface,"  says  Professor  Young,  "  almost 
compel  the  supposition  that  there  is  a  crust  of  some  kind  which  re- 
strains the  imprisoned  gases,  and  through  which  they  force  their  way 
with  great  violence.  This  crust  may  consist  of  a  more  or  less  con- 
tinuous sheet  of  rain — not  of  water,  of  course,  but  of  materials  whose 
vapours  are  shown  by  means  of  the  spectroscope  to  exist  in  the  solar 
atmosphere,  and  whose  condensations  and  combinations  are  supposed 
to  furnish  the  solar  heat.  The  continuous  outflow  of  the  solar  heat  is 
equivalent  to  the  supply  that  would  be  developed  by  the  condensation 
from  steam  to  vapour  of  a  layer  about  five  feet  thick  over  the  whole 
surface  of  the  sun  per  minute.  As  this  tremendous  rain  descends,  the 
velocity  of  the  falling  drops  would  be  increased  by  the  resistance  of  the 
dense  gases  underneath,  the  drops  would  increase  until  continuous 
sheets  would  be  formed;  and  the  sheets  would  unite  and  form  a  sort 
of  bottomless  ocean,  resting  upon  the  compressed  vapours  beneath  and 
pierced  by  innumerable  ascending  jets  and  bubbles.  It  would  have 
nearly  a  constant  depth  in  thickness,  because  it  would  re-evaporate  at 
the  bottom  nearly  as  fast  as  it  would  grow  by  the  descending  rains 
above,  though  probably  the  thickness  of  this  sheet  would  continually 
increase  at  some  slow  rate,  and  its  whole  diameter  diminish.  In  other 
words,  the  sun,  according  to  this  view,  is  a  gigantic  bubble,  whose 
walls  are  gradually  thickening  and  its  diameter  diminishing  at  a  rate 
determined  by  its  loss  of  heat.  It  differs,  however,  from  ordinary 
bubbles  in  the  fact  that  its  skin  is  constantly  penetrated  by  blasts  and 
jets  from  within." 


THE  PAST  AND  FUTURE  OF  OUR  EARTH     69 

becoming  less,  the  whole  earth  contracting  until  it  became 
entirely  liquid;  or  rather  these  changes  would  progress 
until  no  considerable  portion  of  the  earth  would  be  gaseous, 
for  doubtless  long  before  this  stage  was  reached  large  por- 
tions of  the  earth  would  have  become  solid.  As  to  the 
position  which  the  solid  parts  of  the  earth's  globe  would 
assume  when  the  first  processes  of  solidification  took  place, 
we  must  not  fall  into  the  mistake  of  judging  from  the  for- 
mation of  a  crust  of  ice  on  freezing  water  that  these  solid 
parts  would  form  a  crust  upon  the  earth.  Water  presents 
an  exception  to  other  substances,  in  being  denser  in  the 
liquid  form  than  as  a  solid.  Some  metals  and  alloys  are 
like  water  in  this  respect;  but  with  most  earthy  substances, 
"  and  notably,"  says  Dr.  Sterry  Hunt,  "  the  various  min- 
erals and  earthy  compounds  like  those  which  may  be  sup- 
posed to  have  made  up  the  mass  of  the  molten  globe,  the 
case  is  entirely  different.  The  numerous  and  detailed  ex- 
periments of  Sainte-Claire  Deville,  and  those  of  Delesse,  be- 
sides the  earlier  ones  of  Bischof,  unite  in  showing  that  the 
density  of  fused  rocks  is  much  less  than  that  of  the  crystal- 
line products  resulting  from  their  slow  cooling,  these  being, 
according  to  Deville,  from  one  seventh  to  one  sixteenth 
heavier  than  the  fused  mass,  so  that  if  formed  at  the  surface 
they  would,  in  obedience  to  the  laws  of  gravity,  tend  to 
sink  as  soon  as  formed."  9 

Nevertheless,  inasmuch  as  solidification  would  occur  at 
the  surface,  where  the  radiation  of  heat  would  take  place 
most  rapidly,  and  as  the  descending  solid  matter  would 
be  gradually  liquefied,  it  seems  certain  that  for  a  long  time 
the  solid  portions  of  the  earth,  though  not  forming  a  solid 
crust,  would  occupy  the  exterior  parts  of  the  earth's  globe. 
After  a  time,  the  whole  globe  would  have  so  far  cooled  that 
a  process  of  aggregation  of  solid  matter  around  the  centre 
of  the  earth  would  take  place.  The  matter  so  aggregated 
consisted  probably  of  metallic  and  metalloidal  compounds 
denser  than  the  material  forming  the  crust  of  the  earth. 

9  It  is  as  yet  doubtful  how  far  the  recent  experiments  of  Mallet 
affect  this  reasoning. 


70  PROCTOR 

Between  the  solid  centre  and  the  solidifying  crust  there 
would  be  a  shell  of  uncongealed  matter,  gradually  dimin- 
ishing in  amount,  but  a  portion  probably  retaining  its  liquid 
condition  even  to  the  present  time,  whether  existing  in 
isolated  reservoirs,  or  whether,  as  Scrope  opines,  it  forms 
still  a  continuous  sheet  surrounding  the  solid  nucleus.  One 
strange  fact  of  terrestrial  magnetism  may  be  mentioned  in 
partial  confirmation  of  the  theory  that  the  interior  of  the 
earth  is  of  this  nature — a  great  solid  mass,  separated  from 
the  solid  crust  by  a  viscous  plastic  ocean:  the  magnetic 
poles  of  the  earth  are  changing  in  position  in  a  manner 
which  seems  only  explicable  on  the  supposition  that  there 
is  an  interior  solid  globe  rotating  under  the  outer  shell,  but 
at  a  slightly  different  rate,  gaining  or  losing  one  complete 
rotation  in  the  course  of  about  six  hundred  and  fifty  years. 
Be  this  as  it  may,  we  find  in  this  theory  an  explanation 
of  the  irregularities  of  the  earth's  surface.  The  solid  crust, 
contracting  at  first  more  rapidly  than  the  partially  liquid 
mass  within,  portions  of  this  liquid  matter  would  force 
their  way  through  and  form  glowing  oceans  outside  the 
crust.  Geology  tells  us  of  regions  which,  unless  so  formed, 
must  have  been  produced  in  the  much  more  startling  man- 
ner conceived  by  Meyer,  who  attributed  them  to  great 
meteoric  downfalls.10  At  a  later  stage,  when  the  crust, 

10  There  is  very  little  new  under  the  sun.  In  dealing  with  the  mul- 
titudinous lunar  craters,  which  were  certainly  formed  in  ages  when  un- 
attached meteors  were  enormously  greater  in  number  and  size  than 
at  present,  I  mentioned  as  a  consideration  not  to  be  overlooked  the 
probability  that  some  of  the  meteoric  matter  falling  on  the  moon  when 
she  was  plastic  with  intensity  of  heat  might  be  expected  to  leave  traces 
which  we  could  discern;  and  although  none  of  the  larger  lunar  craters 
could  be  so  formed,  yet  some  of  the  smaller  craters  in  these  lunar  re- 
gions where  craters  overlap  like  the  rings  left  by  raindrops  which  have 
fallen  on  a  plastic  surface,  might  be  due  to  meteoric  downfall.  I  find 
that  Meyer  had  far  earlier  advanced  a  similar  idea  in  explanation  of 
those  extensive  regions  of  our  earth  which  present  signs  of  having 
been  in  a  state  of  igneous  fluidity.  Again,  two  or  three  years  ago,  Sir 
W.  Thomson  startled  us  all  by  suggesting  the  possibility  that  vegetable 
life  might  have  been  introduced  upon  our  earth  by  the  downfall  of 
fragments  of  old  worlds.  Several  years  before,  Dr.  Sterry  Hunt  had 
pointed  to  evidence  which  tends  to  show  that  large  meteoric  globes 
had  fallen  on  the  earth,  and  he  showed  further  that  some  meteors  con- 


THE  PAST  AND  FUTURE  OF  OUR  EARTH    71 

having  hitherto  cooled  more  rapidly  than  the  interior,  be- 
gan to  have  a  slower  rate  of  cooling,  the  retreating  nucleus 
left  the  crust  to  contract  upon  it,  corrugating  in  the  process, 
and  so  forming  the  first  mountain  ranges  upon  the  sphe- 
roidal earth,  which  preceding  processes  had  left  partially 
deformed  and  therefore  ready  to  become  in  due  time 
divided  into  oceans  and  continents. 

At  this  stage  the  earth  must  have  been  surrounded  by 
an  atmosphere  much  denser  than  that  now  existing,  and 
more  complex  in  constitution.  We  may  probably  form 
the  most  trustworthy  opinion  of  the  nature  of  the  earth's 
atmosphere  and  the  probable  condition  of  the  earth's  sur- 
face at  this  early  epoch  by  following  the  method  of  reason- 
ing employed  by  Dr.  Sterry  Hunt.  It  will  be  remembered 
that  he  conceives  an  intense  heat  applied  to  the  earth  as  at 
present  existing,  and  infers  the  chemical  results.  It  is  evi- 
dent that  such  a  process  would  result  in  the  oxidation  of 
every  form  of  carbonaceous  matter;  all  carbonates,  chlo- 
rides, and  sulphates  would  be  converted  into  silicates — 
carbon,  chlorine,  and  sulphur  being  separated  in  the  form 
of  acid  gases.  These  gases,  with  nitrogen,  an  excess  of 
oxygen,  and  enormous  quantities  of  aqueous  vapour,  would 
form  an  atmosphere  of  great  density.  In  such  an  atmos- 
phere condensation  would  only  take  place  at  a  temperature 
far  above  the  present  boiling  point;  and  the  lower  level  of 
the  slowly  cooling  crust  would  be  drenched  with  a  heated 
solution  of  hydrochloric  acid,  whose  decomposing  action, 
aided  by  its  high  temperature,  would  be  exceedingly  rapid. 
The  primitive  igneous  rock  on  which  these  heavy  showers 
fell  probably  resembled  in  composition  certain  furnace- 
slags  or  basic  volcanic  glasses.  Chlorides  of  the  various 

tain  hydrocarbons  and  certain  metallic  compounds  indicating  processes 
of  vegetation.  Dr.  Hunt  tells  me  that,  in  his  opinion,  some  of  the 
meteors  whose  fragments  have  fallen  on  the  earth  in  historic  times 
were  once  covered  with  vegetation,  since  otherwise,  according  to  our 
present  chemical  experience,  the  actual  condition  of  these  meteoric 
fragments  would  be  inexplicable.  He  does  not  regard  them  as  frag- 
ments of  a  considerable  orb  comparable  even  with  the  least  of  the 
planets,  but  still,  whatever  their  dimensions  may  have  been,  he  con- 
siders that  vegetable  life  must  have  formerly  existed  upon  them. 


72  PROCTOR 

bases  would  be  formed,  and  silica  would  be  separated  under 
the  decomposing  action  of  the  heated  showers  until  the 
affinities  of  the  hydrochloric  acid  were  satisfied.  Later,  sul- 
phuric acid  would  be  formed  in  large  quantities  by  the  com- 
bination of  oxygen  with  the  sulphurous  acid  of  the  primeval 
atmosphere.  After  the  compounds  of  sulphur  and  chlorine 
had  been  separated  from  the  air,  carbonic  acid  would  still 
continue  to  be  an  important  constituent  of  the  atmosphere. 
This  constituent  would  gradually  be  diminished  in  quantity, 
during  the  conversion  of  the  complex  aluminous  silicates 
into  hydrated  silicate  of  alumina,  or  clay,  while  the  sepa- 
rated lime,  magnesia,  and  alkalies  would  be  changed  into 
bicarbonates,  and  carried  down  to  the  sea  in  a  state  of 
solution. 

Thus  far  the  earth  was  without  life;  at  least  no  forms 
of  life,  vegetable  or  animal,  with  which  we  are  familiar, 
could  have  existed  while  the  processes  hitherto  described 
were  taking  place.  The  earth  during  the  long  series  of  ages 
required  for  these  changes  was  in  a  condition  comparable 
with  the  condition  through  which  Jupiter  and  Saturn  are 
apparently  at  present  passing.  A  dense  atmosphere  con- 
cealed the  surface  of  the  earth,  even  as  the  true  surface  of 
Jupiter  is  now  concealed.  Enormous  cloud-masses  were 
continually  forming  and  continually  pouring  heavy  showers 
on  the  intensely  heated  surface  of  the  planet,  throughout 
the  whole  of  the  enormous  period  which  elapsed  between 
the  time  when  first  the  earth  had  a  surface,  and  the  time 
when  the  atmosphere  began  to  resemble  in  constitution  the 
air  we  breathe.  Even  when  vegetable  life,  such  as  we  are 
familiar  with,  was  first  possible,  the  earth  was  still  intensely 
heated,  and  the  quantity  of  aqueous  vapour  and  cloud 
always  present  in  the  air  must  have  been  far  greater  than  at 
present. 

It  has  been  in  vain,  thus  far,  that  men  have  attempted 
to  lift  the  veil  which  conceals  the  beginning  of  life  upon 
the  earth.  It  would  not  befit  me  to  express  an  opinion  on 
the  controversy  whether  the  possibility  of  spontaneous 
generation  has,  or  has  not,  been  experimentally  verified. 


THE  PAST  AND  FUTURE  OF  OUR  EARTH 


73 


That  is  a  question  on  which  experts  alone  can  give  an 
opinion  worth  listening  to;  and  all  that  can  here  be  noted 
is  that  experts  are  not  agreed  upon  the  subject.  As  a  mere 
speculation  it  may  be  suggested  that,  somewhat  as  the  ele- 
ments when  freshly  released  from  chemical  combination 
show  for  a  short  time  an  unusual  readiness  to  enter  into 
new  combinations,  so  it  may  be  possible  that,  when  the 
earth  was  fresh  from  the  baptism  of  liquid  fire  to  which  her 
primeval  surface  had  for  ages  been  exposed,  certain  of  the 
substances  existing  on  her  surface  were  for  the  time  in  a 
condition  fitting  them  to  pass  to  a  higher  order  of  exist- 
ence, and  that  then  the  lower  forms  of  life  sprang  spontane- 
ously into  existence  on  the  earth's  still  throbbing  bosom.  In 
any  case,  we  need  not  feel  hampered  by  religious  scruples  in 
considering  the  possibility  of  the  spontaneous  generation  of 
life  upon  the  earth.  It  would  be  straining  at  a  gnat  and  swal- 
lowing a  camel  if  we  found  a  difficulty  of  that  sort  here, 
after  admitting,  as  we  are  compelled  by  clearest  evidence 
to  admit,  the  evolution  of  the  earth  itself  and  of  the  system 
to  which  the  earth  belongs,  by  purely  natural  processes. 
The  student  of  science  should  view  these  matters  apart 
from  their  supposed  association  with  religious  questions, 
apart  in  particular  from  interpretations  which  have  been 
placed  upon  the  Bible  records.  We  may  be  perfectly  satis- 
fied that  the  works  of  God  will  teach  us  aright  if  rightly 
studied.  Repeatedly  it  has  been  shown  that  ideas  respect- 
ing creation  which  had  come  to  be  regarded  as  sacred  be- 
cause they  were  ancient,  were  altogether  erroneous,  and  it 
may  well  be  so  in  this  matter  of  the  creation  of  life.11 

u  It  is  not  for  me  to  undertake  to  reconcile  the  Bible  account  of 
creation  with  the  results  which  science  is  bringing  gradually  more 
clearly  before  us.  It  seems  to  me  unfortunate,  in  fact,  that  such 
reconciliation  should  be  thought  necessary.  But  it  must  be  conceded, 
I  suppose,  by  all,  that  it  is  not  more  difficult  to  reconcile  modern  bio- 
logical theories  of  evolution  with  the  Bible  record  than  it  is  to  recon- 
cile with  that  record  the  theory  of  the  evolution  of  the  solar  system. 
Yet  strangely  enough  many  oppose  the  biological  theories  (not  without 
anger),  who  readily  admit  that  some  form  or  other  of  the  nebular 
hypothesis  of  the  solar  system  must  be  adopted  in  order  to  explain  the 
peculiarities  of  structure  presented  by  that  system. 


74  PROCTOR 

Whatever  opinion  we  form  on  these  points,  it  seems 
probable  that  vegetable  life  existed  on  the  earth  before  ani- 
mal life,  and  also  that  primeval  vegetation  was  far  more 
luxuriant  than  the  vegetation  of  our  own  time.  Vast  for- 
ests were  formed,  of  which  our  coal-fields,  enormous  as  is 
their  extent,  represent  merely  a  small  portion  preserved  in 
their  present  form  through  a  fortuitous  combination  of  ex- 
ceptional conditions.  By  far  the  greater  portion  of  those 
forest  masses  underwent  processes  of  vegetable  decay 
effectually  removing  all  traces  of  their  existence.  What 
escaped,  however,  suffices  to  show  the  amazing  luxuriance 
with  which  vegetation  formerly  throve  over  the  whole 
earth. 

In  assuming  the  probability  that  vegetable  life  preceded 
animal  life,  I  may  appear  to  be  opposing  myself  to  an  ac- 
cepted paleontological  doctrine,  according  to  which  animal 
and  vegetable  life  began  together  upon  the  earth.  But  I 
would  remind  you  that  the  actual  teaching  of  the  ablest, 
and  therefore  the  most  cautious,  paleontologists  on  this 
point,  amounts  merely  to  this,  that  if  the  geological  record 
as  at  present  known  be  assumed  to  be  coeval  with  the 
commencement  of  life  upon  the  globe,  then  animals  and 
plants  began  their  existence  together.  In  a  similar  way  the 
teachings  of  geology  and  paleontology  as  to  the  nature  of 
the  earliest  known  forms  of  life  and  as  to  the  succession 
of  faunae  and  florae,  depend  on  an  admittedly  imperfect 
record.  Apart,  however,  from  this  consideration,  I  do  not 
think  it  would  serve  any  useful  purpose  if  I  were  to  at- 
tempt, I  will  not  say  to  discuss,  for  that  is  out  of  the  ques- 
tion, but  to  speak  of  the  geological  evidence  respecting 
that  portion  of  the  past  history  of  our  earth  which  belongs 
to  the  interval  between  the  introduction  of  life  upon  the 
surface  and  the  present  time.  In  particular,  my  opinion 
on  the  interesting  question,  whether  all  the  forms  of  life 
upon  the  earth,  including  the  various  races  of  man,  came 
into  being  by  processes  of  evolution,  could  have  no  weight 
whatever.  I  may  remark  that,  even  apart  from  the  evi- 
dence which  the  most  eminent  biologists  have  brought  to 


THE  PAST  AND  FUTURE  OF  OUR  EARTH    75 

bear  on  this  question,  it  seems  to  me  illogical  to  accept 
evolution  as  sufficient  to  explain  the  history  of  our  earth 
during  millions  of  years  prior  to  the  existence  of  life,  and 
to  deny  its  sufficiency  to  explain  the  development  of  life 
(if  one  may  so  speak),  upon  the  earth.  It  seems  even  more 
illogical  to  admit  its  operation  up  to  any  given  stage  in  the 
development  of  life,  and  there  to  draw  a  hard-and-fast  line 
beyond  which  its  action  can  not  be  supposed  to  have  ex- 
tended.12 Nor  can  I  understand  why  it  should  be  con- 
sidered a  comforting  thought,  that  at  this  or  that  epoch 
in  the  history  of  the  complex  machine  of  life,  some  imper- 
fection in  the  machinery  compelled  the  intervention  of 
God — thus  presented  to  our  contemplation  as  Almighty, 
but  very  far  from  being  All-wise. 

There  is,  however,  one  aspect  in  which  the  existence  of 
life  has  to  be  considered  as  intimately  associated  with  the 
future  history  of  our  earth.  We  perceive  that  the  abun- 
dance of  primeval  vegetation  during  long  ages,  aided  by 
other  processes  tending  gradually  to  reduce  the  amount 
of  carbonic-acid  gas  in  the  air,  must  have  led  to  a  gradual 
change  in  the  constitution  of  the  atmosphere.  At  a  later 
epoch,  when  animal  life  and  vegetable  life  were  more 
equally  proportioned,  a  state  of  things  existed  which,  so 
far  as  can  be  judged,  might  have  lasted  many  times  as  long 
as  it  has  already  lasted  had  not  man  appeared  upon  the 
scene.  But  it  seems  to  me  impossible  to  consider  what  is 
actually  taking  place  on  the  earth  at  present,  without  per- 
ceiving that  within  periods,  short  indeed  by  comparison 
with  geological  eras,  and  still  shorter  compared  with  the 
intervals  to  which  the  astronomical  history  of  our  earth  has 
introduced  us,  the  condition  of  the  earth  as  an  abode  of 

13  Since  I  thus  spoke,  a  new  and  as  it  seems  to  me  an  even  more 
illogical  limit  has  been  suggested  for  the  operation  of  the  process  of 
evolution  as  affecting  the  development  of  life,  and  this  by  an  advocate 
of  the  general  doctrine  of  evolution.  I  refer  to  the  opinion  advanced 
by  Mr.  J.  Fiske,  of  Harvard  University,  that  "  no  race  of  organisms 
can  in  future  be  produced  through  the  agency  of  natural  selection 
and  direct  adaptation,  which  shall  be  zoologically  distinct  from,  and 
superior  to,  the  human  race." 


76  PROCTOR 

life  will  be  seriously  modified  by  the  ways  and  works  of 
man.  It  is  only  in  the  savage  state  that  man  is  content  to 
live  upon  the  produce  of  the  earth,  taking  his  share,  as  it 
were,  of  what  the  earth  (under  the  fruitful  heat  of  the  sun, 
which  is  her  life)  brings  forth — day  by  day,  month  by 
month,  year  by  year,  and  century  by  century.  But  civilized 
man  is  not  content  to  take  his  share  of  the  earth's  income, 
he  uses  the  garnered  wealth  which  is  the  earth's  capital — 
and  this  at  a  rate  which  is  not  only  ever  increasing,  but  is 
increasing  at  an  increasing  rate.  The  rapid  consumption 
of  coal  is  but  a  single  instance  of  his  wasteful  expenditure 
of  the  stores  which  during  countless  ages  have  been 
gathered  together,  seemingly  for  the  use  of  man.  In  this 
country  (America),  I  need  not  dwell  upon  the  fact  that, 
in  many  other  ways,  man  is  consuming,  if  not  wasting, 
supplies  of  earth-wealth  which  can  not  be  replaced.  It  is 
not  merely  what  is  found  within  the  earth,  but  the  store  of 
wealth  which  clothes  the  earth's  surface,  which  is  thus  be- 
ing exhausted.  Your  mighty  forests  seem  capable  of  sup- 
plying all  the  timber  that  the  whole  race  of  man  could 
need  for  ages;  yet  a  very  moderate  computation  of  the  rate 
at  which  they  are  being  cut  down,  and  will  presumably 
continue  to  be,  by  a  population  increasing  rapidly  in  num- 
bers and  in  the  destructive  capabilities  which  characterize 
modern  civilization,  would  show  that  America  will  be  de- 
nuded of  its  forest-wealth  in  about  the  same  period  which 
we  in  England  have  calculated  as  probably  limiting  the 
effective  duration  of  our  stores  of  coal.  That  period — a 
thousand  or  twelve  hundred  years — may  seem  long  com- 
pared with  the  life  of  individual  men,  long  even  compared 
with  the  duration  of  any  nation  in  the  height  of  power; 
but  though  men  and  nations  pass  away,  the  human  race 
continues,  and  a  thousand  years  are  as  less  than  a  day  in 
the  history  of  that  race.  Looking  forward  to  that  future 
day,  seemingly  so  remote,  but  (on  the  scale  upon  which 
we  are  at  present  tracing  our  earth's  history)  in  reality  the 
to-morrow  of  our  earth,  we  see  that  either  a  change  in 
their  mode  of  civilization  will  be  forced  on  the  human  race, 


THE  PAST  AND  FUTURE  OF  OUR  EARTH     77 

or  else  it  will  then  have  become  possible,  as  your  Ericsson 
has  already  suggested,  to  make  the  sun's  daily  heat  the 
mainspring  of  the  machinery  of  civilization. 

But  turning  from  those  portions  of  the  past  and  future 
of  our  earth  which,  by  comparison  with  the  astronomical 
eras  of  her  history,  may  be  regarded  as  present,  let  us  con- 
sider, so  far  as  known  facts  permit,  the  probable  future  of 
the  earth  after  astronomical  eras  comparable  with  those 
which  were  presented  to  us  when  we  considered  her  past 
history. 

One  of  the  chief  points  in  the  progression  of  the  earth 
toward  her  present  condition  was  the  gradual  passing  away 
of  the  heat  with  which  formerly  her  whole  globe  was  in- 
stinct. We  have  now  to  consider  whether  this  process  of 
cooling  is  still  going  on,  and  how  far  it  is  likely  to  ex- 
tend. In  this  inquiry  we  must  not  be  misled  by  the  prob- 
able fact,  for  such  it  seems,  that  during  hundreds  of  thou- 
sands of  years  the  general  warmth  of  the  surface  of  the 
earth  has  not  appreciably  diminished.  In  the  first  place, 
hundreds  of  thousands  of  years  are  the  seconds  of  the  time- 
measures  we  have  now  to  deal  with;  and  next,  it  is  known 
that  the  loss  of  temperature  which  our  earth  is  at  present 
undergoing  chiefly  affects  the  interior  parts  of  her  globe. 
The  inquiries  of  Mallet  and  others  show  that  the  present 
vulcanian  energies  of  the  earth  are  due  in  the  main  to  the 
gradual  withdrawal  of  the  earth's  nuclear  parts  from  the 
surface  crust,  because  of  the  relatively  more  rapid  loss  of 
heat  by  the  former.  The  surface  crust  is  thus  left  to  con- 
tract under  the  action  of  gravity,  and  vulcanian  phenomena 
— that  is,  volcanoes  and  earthquakes — represent  the  me- 
chanical equivalent  of  this  contraction.  Here  is  a  process 
which  can  not  continue  forever,  simply  because  it  is  in  its 
very  nature  exhaustive  of  the  energy  to  which  it  is  due.  It 
shows  us  that  the  earth's  nuclear  regions  are  parting  with 
their  heat,  and  as  they  can  not  part  with  their  heat  without 
warming  the  surface-crust,  which  nevertheless  grows  no 
wanner,  we  perceive  that  the  surface  heat  is  maintained 
from  a  source  which  is  being  gradually  exhausted.  The 


78  PROCTOR 

fitness  of  the  earth  to  be  the  abode  of  life  will  not  only 
be  affected  directly  in  this  way,  but  will  be  indirectly 
affected  by  the  loss  of  that  vulcanian  energy  which  appears 
to  be  one  of  its  necessary  conditions.  At  present,  the  sur- 
face of  the  earth  is  like  the  flesh  clothing  the  living  body; 
it  does  not  wear  out  because  (through  the  life  which  is 
within  it)  it  undergoes  continual  change.  But  even  as  the 
body  itself  is  consumed  by  natural  processes  so  soon  as  life 
has  passed  from  it,  so,  when  the  internal  heat  of  the  earth, 
which  is  its  life,  shall  have  passed  away,  her  surface  will 
"  grow  old  as  doth  a  garment  ";  and  with  this  inherent  ter- 
restrial vitality  will  pass  away  by  slow  degrees  the  life 
which  is  upon  the  earth. 

In  dealing  with  the  past  history  of  our  earth,  we  recog- 
nised a  time  when  she  was  a  sun,  rejoicing  as  a  giant  in 
the  strength  of  youth;  and  later  we  considered  a  time  when 
her  condition  resembled  that  of  the  planets  Jupiter  and 
Saturn,  whose  dense  atmospheres  seem  to  be  still  loaded 
with  the  waters  which  are  to  form  the  future  oceans  of  those 
noble  orbs.  In  considering  our  earth's  future,  we  may 
recognise  in  the  moon's  actual  condition  a  stage  through 
which  the  earth  will  hereafter  have  to  pass.  When  the  earth's 
inherent  heat  has  passed  away  and  long  ages  have  elapsed 
since  she  had  been  the  abode  of  life,  we  may  believe  that  her 
desert  continents  and  frost-bound  oceans  will  in  some  de- 
gree resemble  the  arid  wastes  which  the  astronomer  rec- 
ognises in  the  lunar  surface.  And  yet  it  is  not  to  be  sup- 
posed that  the  appearance  of  the  earth  will  ever  be  closely 
similar  to  that  presented  by  the  moon.  The  earth  may 
part,  as  completely  as  the  moon  has,  with  her  internal  heat; 
the  rotation  of  the  earth  may  in  hundreds  of  millions  of 
years  be  slowed  down  by  tidal  action  into  agreement  with 
the  period  in  which  the  moon  completes  her  monthly  orbit; 
and  every  form  of  animal  and  vegetable  life  may  perish 
from  off  the  face  of  the  earth:  yet  ineffaceable  traces  of  the 
long  ages  during  which  her  surface  was  clothed  with  life 
and  instinct  with  inherent  vitality,  will  distinguish  her 
from  the  moon,  where  the  era  of  life  was  incomparably 


THE  PAST  AND  FUTURE  OF  OUR  EARTH     79 

shorter.  Even  if  the  speculations  of  Stanislas  Meunier  be 
just,  according  to  which  the  oceans  will  gradually  be  with- 
drawn beneath  the  surface  crust  and  even  the  atmosphere 
almost  wholly  disappear,  there  would  forever  remain  the 
signs  of  changes  brought  about  by  rainfall  and  snowfall,  by 
wind  and  storm,  by  river  and  glacier,  by  ocean  waves  and 
ocean  currents,  by  the  presence  of  vegetable  life  and  of 
animal  life  during  hundreds  of  millions  of  years,  and  even 
more  potently  by  the  fiery  deluge  poured  continually  on 
the  primeval  surface  of  our  globe.  By  all  these  causes  the 
surface  of  the  earth  has  been  so  wrought  upon  as  no  longer 
to  resemble  the  primary  igneous  rock  which  we  seem  to 
recognise  in  the  scarred  surface  of  our  satellite. 

Dare  we  look  onward  to  yet  later  stages  in  the  history 
of  our  earth?  Truly  it  is  like  looking  beyond  death;  for 
now  imagination  presents  our  earth  to  us  as  an  inert  mass, 
not  only  lifeless  as  at  the  beginning,  but  no  longer  pos- 
sessing that  potentiality  of  life  which  existed  in  her  sub- 
stance before  life  appeared  upon  her  surface.  We  trace  her 
circling  year  after  year  around  the  sun,  serving  no  useful 
purpose  according  to  our  conceptions.  The  energy  repre- 
sented by  her  motions  of  rotation  and  revolution  seems  to 
be  as  completely  wasted  as  are  those  parts  (the  whole  save 
only  one  23O,ooo,oooth  portion)  of  the  sun's  light  and  heat, 
which,  falling  on  no  planet,  seem  to  be  poured  uselessly  into 
desert  space.  Long  as  has  been,  and  doubtless  will  be,  the 
duration  of  life  upon  the  earth,  it  seems  less  than  a  second 
of  time  compared  with  those  two  awful  time-intervals — one 
past,  when  as  yet  life  had  not  begun,  the  other  still  to  come, 
when  all  life  shall  have  passed  away. 

But  we  are  thus  led  to  contemplate  time-intervals  of  a 
yet  higher  order — to  consider  the  eras  belonging  to  the 
lifetime  of  the  solar  system  itself.  Long  after  the  earth 
shall  have  ceased  to  be  the  abode  of  life  other  and  nobler 
orbs  will  become  in  their  time  fit  to  support  millions  of  forms 
as  well  of  animal  as  of  vegetable  existence;  and  the  later 
each  planet  is  in  thus  "  putting  on  life,"  the  longer  will  be 
the  duration  of  the  life-supporting  era  of  its  own  existence. 


80  PROCTOR 

Even  those  time-intervals  will  pass,  however,  until  every 
orb  in  turn  has  been  the  scene  of  busy  life,  and  has  then, 
each  after  its  due  life-season,  become  inert  and  dead.  One 
orb  alone  will  then  remain,  on  which  life  will  be  possible — 
the  sun,  the  source  whence  life  had  been  sustained  in  all 
those  worlds.  And  then,  after  the  lapse,  perchance,  of  a 
lifeless  interval,  compared  with  which  all  the  past  eras  of  the 
solar  system  were  utterly  insignificant,  the  time  will  arrive 
when  the  sun  will  be  a  fit  abode  for  living  creatures.  There- 
after, during  ages  infinite  to  our  conceptions,  the  great 
central  orb  will  be  (as  now,  though  in  another  sense)  the 
life  of  the  solar  system.  We  may  even  look  onward  to 
still  more  distant  changes,  seeing  that  the  solar  system  is 
itself  moving  "on  an  orbit,  though  the  centre  round  which 
it  travels  is  so  distant  that  as  yet  it  remains  unknown. 
We  see  in  imagination  change  after  change,  cycle  after 
cycle,  till 

"  Drawn  on  paths  of  never-ending  duty, 

The  worlds — eternity  begun — 
Rest,  absorbed  in  ever-glorious  beauty, 
On  the  Heart  of  the  All-Central  Sun." 

But  in  reality  it  is  only  because  our  conceptions  are 
finite  that  we  thus  look  forward  to  an  end  even  as  we  seek 
to  trace  events  back  to  a  beginning.  The  notion  is  incon- 
ceivable to  us  that  absolutely  endless  series  of  change  may 
take  place  in  the  future  and  have  taken  place  in  the  past; 
equally  inconceivable  is  the  notion  that  series  on  series  of 
material  combinations,  passing  onward  to  ever-higher 
orders — from  planets  to  suns,  from  suns  to  sun-systems, 
from  sun-systems  to  galaxies,  from  galaxies  to  systems  of 
galaxies,  from  these  to  higher  and  higher  orders,  abso- 
lutely without  end — may  surround  us  on  every  hand.  And 
yet,  as  I  set  out  by  saying,  these  things  are  not  more  in- 
conceivable than  infinity  of  time  and  infinity  of  space,  while 
the  idea  that  time  and  space  are  finite  is  not  merely  incon- 
ceivable, but  opposed  directly  to  what  the  mind  conceives 
of  space  and  time.  It  has  been  said  that  progression  neces- 
sarily implies  a  beginning  and  an  end;  but  this  is  not  so 


THE  PAST  AND  FUTURE  OF  OUR  EARTH     8l 

where  the  progression  relates  to  absolute  space  or  time. 
No  one  can  indeed  doubt  that  progression  in  space  is  of  its 
very  nature  limitless.  But  this  is  equally  true,  though  not 
less  inconceivable,  of  time.  Progression  implies  only  rela- 
tive beginning  and  relative  ending;  but  that  there  should 
be  an  absolute  beginning  or  an  absolute  end  is  not  merely 
inconceivable,  like  absolute  eternity,  but  is  inconsistent 
with  the  necessary  conditions  of  the  progression  of  time  as 
presented  to  us  by  our  conceptions.  Those  who  can  may 
find  relief  in  believing  in  absolutely  void  space  and  abso- 
lutely unoccupied  time  before  some  very  remote  but  not 
infinitely  remote  epoch,  which  may  in  such  belief  be  called 
the  beginning  of  all  things;  but  the  void  time  before  that  be- 
ginning can  have  had  no  beginning,  unless  it  were  preceded 
by  time  not  unoccupied  by  events,  which  is  inconsistent 
with  the  supposition.  We  find  no  absolute  beginning  if  we 
look  backward;  and  looking  forward  we  not  only  find  an 
absolute  end  inconceivable  by  reason,  but  revealed  religion 
— as  ordinarily  interpreted — teaches — that  on  that  side  lies 
an  eternity  not  of  void  but  of  occupied  time.  The  time- 
intervals,  then,  which  have  presented  themselves  to  our  con- 
templation in  dealing  with  the  past  and  future  of  our  earth, 
being  in  their  nature  finite,  however  vast,  are  less  than  the 
shortest  instant  in  comparison  with  absolute  time,  which — 
endless  itself — is  measured  by  endless  cycles  of  change. 
And  in  like  manner,  the  space  seemingly  infinite  from  which 
our  solar  system  has  drawn  its  materials — in  other  words, 
the  universe  as  partially  revealed  to  us  in  the  study  of  the 
star-depths — is  but  the  merest  point  by  comparison  with 
absolute  space.  The  end,  seemingly  so  remote,  to  which 
our  earth  is  tending,  the  end  infinitely  more  remote  to 
which  the  solar  system  is  tending,  the  end  of  our  galaxy, 
the  end  of  systems  of  such  galaxies  as  ours — all  these  end- 
ings (each  one  of  which  presents  itself  in  turn  to  our  con- 
ceptions as  the  end  of  the  universe  itself)  are  but  the  begin- 
nings of  eras  comparable  with  themselves,  even  as  the  be- 
ginnings to  which  we  severally  trace  back  the  history  of 
our  planet,  of  the  planetary  system,  and  of  galaxies  of  such 
6 


82  PROCTOR 

systems,  are  but  the  endings  of  prior  conditions  which 
have  followed  each  other  in  infinite  succession.  The  wave 
of  life  which  is  now  passing  over  our  earth  is  but  a  ripple 
in  the  sea  of  life  within  the  solar  system;  this  sea  of  life 
is  itself  but  as  a  wavelet  on  the  ocean  of  eternal  life  through- 
out the  universe.  Inconceivable,  doubtless,  are  these  in- 
finities of  time  and  space,  of  matter,  of  motion,  and  of 
life.  Inconceivable  that  the  whole  universe  can  be  for  all 
time  the  scene  of  the  operation  of  infinite  personal  power, 
omnipresent,  all-knowing.  Utterly  incomprehensible  how 
Infinite  Purpose  can  be  associated  with  endless  material 
evolution.  But  it  is  no  new  thought,  no  modern  discovery, 
that  we  are  thus  utterly  powerless  to  conceive  or  compre- 
hend the  idea  of  an  Infinite  Being,  Almighty,  All-knowing, 
Omnipresent,  and  Eternal,  of  whose  inscrutable  purpose 
the  material  universe  is  the  unexplained  manifestation. 
Science  is  in  presence  of  the  old,  old  mystery;  the  old,  old 
questions  are  asked  of  her:  "  Canst  thou  by  searching  find 
out  God?  canst  thou  find  out  the  Almighty  unto  perfec- 
tion? It  is  as  high  as  heaven;  what  canst  thou  do?  deeper 
than  hell;  what  canst  thou  know?  "  And  Science  answers 
these  questions,  as  they  were  answered  of  old,  "  As  touch- 
ing the  Almighty,  we  can  not  find  him  out." 


A   NEW   THEORY  OF   LIFE  IN   OTHER 
WORLDS l 

TWO  opposite  views  have  been  entertained  respecting 
life  in  other  worlds.  One  is  the  theory  which  Brew- 
ster  somewhat  strangely  described  as  the  creed  of 
the  philosopher  and  the  hope  of  the  Christian,  that  nearly 
all  the  orbs  which  people  space  are  the  abode  of  life. 
Brewster,  Chalmers,  Dick,  and  a  host  of  other  writers,  have 
adopted  and  enforced  this  view,  Brewster  going  so  far 
as  to  maintain  the  probability  that  life  may  exist  upon 
the  moon,  dead  though  her  surface  seems,  or  beneath  the 
glowing  photosphere  of  the  sun.  But  even  where  so  ex- 
treme an  opinion  has  not  been  entertained,  the  believers 
in  the  theory  of  a  plurality  of  worlds  have  maintained  that 
all  the  celestial  orbs  have  been  created  to  be,  and  are  at 
this  present  time,  the  abodes  of  life,  or  else  minister  to 
the  wants  of  creatures  living  in  other  orbs.  It  is  worthy 
of  notice  that  this  view  has  been  entertained  even  by 
astronomers,  who,  like  the  Herschels,  have  devoted  their 
lives  to  the  scientific  study  of  the  heavens.  So  completely 
has  the  theory  been  identified,  as  it  were,  with  modern 
astronomy,  that  we  find  the  astronomer  passing  from  a 
statement  respecting  some  observed  fact  about  a  planet,  to 
the  consideration  of  the  bearing  of  the  fact  on  the  require- 
ments of  living  creatures  on  the  planet's  surface,  without 
expressing  any  doubt  whatever  as  to  the  existence  of  such 
creatures.  For  example,  Sir  John  Herschel,  writing  about 
the  rings  of  Saturn,  after  discussing  Lardner's  supposed 
demonstration  that  the  eclipses  caused  by  the  rings  would 
1  From  "  Our  Place  among  Infinities,"  D.  Appleton  and  Company. 

83 


84  PROCTOR 

last  but  for  a  short  time,2  says :  "  This  will  not  prevent, 
however,  some  considerable  regions  of  Saturn  from  suf- 
fering very  long  total  interception  of  the  solar  beams, 
affording  to  our  ideas  but  an  inhospitable  asylum  to  ani- 
mated beings,  ill  compensated  by  the  feeble  light  of  the 
satellites;  but  we  shall  do  wrong  to  judge  of  the  fitness 
or  unfitness  of  their  condition  from  what  we  see  around 
us,  when  perhaps  the  very  combinations  which  convey  to 
our  minds  only  images  of  horror  may  be,  in  reality,  thea- 
tres of  the  most  striking  and  glorious  displays  of  benefi- 
cent contrivance."  And  many  other  such  cases  might 
be  cited. 

Before  passing  to  the  opposite  view  of  life  in  other 
worlds,  a  view  commonly  associated  with  the  name  of  the 
late  Dr.  Whewell,  I  shall  venture  to  quote  a  few  passages 
from  his  "  Bridgewater  Treatise  on  Astronomy  and  Gen- 
eral Physics,"  in  which  he  writes  very  much  like  a  sup- 
porter of  the  theory  he  subsequently  opposed  in  his  "  Plu- 
rality of  Worlds."  Thus,  speaking  of  the  satellites  in  the 
solar  system,  he  says:  "There  is  one  fact  which  imme- 
diately arrests  our  attention;  the  number  of  these  attendant 
bodies  appears  to  increase  as  we  proceed  to  planets  farther 
and  farther  from  the  sun.  Such,  at  least,  is  the  general  rule. 
Mercury  and  Venus,  the  planets  near  the  sun,  have  no  at- 
tendants; the  earth  has  but  one.  Mars,  indeed,  who  is  still 
farther  removed,  has  none,  nor  have  the  minor  planets,  so 
that  the  rule  is  only  approximately  verified.  But  Jupiter, 
who  is  at  five  times  the  earth's  distance,  has  four  satellites; 
and  Saturn,  who  is  again  at  a  distance  nearly  twice  as  great, 
has  seven  "  (now  eight)  "  besides  that  most  extraordinary 
phenomenon,  his  ring,  which  for  purposes  of  illumination  is 
equivalent  to  many  thousand  satellites.  Of  Uranus  it  is 

*  This  is  disproved,  and  the  justice  of  Herschel's  views  demon- 
trated  in  Chapter  VII  of  my  treatise  on  Saturn,  in  which  work  I  give 
a  table  of  the  climatic  relations  in  Saturn  (for  I  also  once  adopted  the 
theory  criticised  above),  the  time  and  place  of  sunrise  and  sunset  in 
Saturnian  latitudes  in  Saturnian  spring,  summer,  autumn,  and  winter, 
and  so  on.  Labour  wasted,  I  fear,  except  as  practice  in  Geometrical 
Astronomy. 


A   NEW  THEORY  OF  LIFE   IN  OTHER  WORLDS      85 

difficult  to  speak,  for  his  great  distance  renders  it  almost 
impossible  to  observe  the  smaller  circumstances  of  his  con- 
dition. It  does  not  appear  at  all  probable  that  he  has  a 
ring  like  Saturn;  but  he  has  at  least  four  satellites  which 
are  visible  to  us,  at  the  enormous  distance  of  nine  hundred 
millions  of  miles,  and  I  believe  that  the  astronomer  will 
hardly  deny  that  he  may  possibly  have  thousands  of  smaller 
ones  circulating  about  him.  But  leaving  conjecture,  and 
taking  only  the  ascertained  cases  of  Venus,  the  earth, 
Jupiter,  and  Saturn,  we  conceive  that  a  person  of  common 
understanding  will  be  strongly  impressed  with  the  persua- 
sion that  the  satellites  are  placed  in  the  system  with  a  view 
to  compensate  for  the  diminished  light  of  the  sun  at  greater 
distances."  Then  he  presently  adds,  after  considering  the 
exceptional  case  of  Mars,  "  No  one  familiar  with  such  con- 
templations will,  by  one  anomaly,  be  driven  from  the  per- 
suasion that  the  end  which  the  arrangements  of  the  satel- 
lites seem  suited  to  answer  is  really  one  of  the  ends  of 
their  creation."  Here  is  the  theory  of  life  in  other  worlds 
definitely  adopted,  and  moreover  presented  in  company 
with  the  extremest  form  of  the  teleological  argument,  and 
that,  too,  by  Whewell,  whose  name  afterward  became  asso- 
ciated with  the  extremest  development  of  the  doctrine  of 
the  paucity  of  worlds ! 

The  Whewellite  theory  is  tolerably  well  known,  though 
certainly  it  is  not  held  in  very  great  favour.  For  my  own 
part,  I  used,  at  one  time,  to  think  that  Whewell  only  ad- 
vanced it  in  jest;  but  now  (perhaps  because  my  own  re- 
searches and  study  have  led  me  to  regard  the  Brewsterian 
theory  as  untenable)  I  recognise  in  Whewell's  later  views 
the  result  of  longer  and  more  careful  study  than  he  had 
given  to  the  subject,  when  (nearly  a  quarter  of  a  century 
earlier)  he  wrote  his  "  Bridgewater  Treatise." 

Whatever  opinion  we  form  as  to  the  theory  advanced 
in  the  "  Plurality  of  Worlds,"  we  must  admit  that  Whewell 
did  good  service  to  science  in  breaking  the  chains  of  old- 
fashioned  ideas,  and  inaugurating  freedom  of  discussion. 
The  stock  writers  on  astronomy  had  been  repeating  so 


86  PROCTOR 

often  the  imperfect  analogies  on  which  astronomers  had 
earlier  insisted,  that  the  suggestions  based  on  such  analogies 
had  come  to  be  regarded  as  so  many  scientific  facts.  The 
earth  is  a  planet,  and  Mars  is  a  planet,  therefore  what  we 
know  about  the  earth  may  be  inferred  respecting  Mars,  no 
account  being  taken  of  the  known  difference  in  the  con- 
dition of  the  two  planets:  accordingly,  not  only  are  the 
white  spots  at  the  Martian  poles  to  be  regarded  as  snow- 
covered  regions,  and  the  blue  markings  on  his  surface  as 
seas,  but  we  are  to  infer  a  similarity  of  climatic  conditions 
and  other  habitudes,  without  entering  into  any  close  con- 
sideration of  the  probable  extent  of  the  planet's  atmos- 
phere, the  heat  received  from  the  sun  by  Mars,  and  a 
variety  of  other  relations  respecting  which  we  are  at  least 
as  well  informed  as  we  are  respecting  the  analogies  in  ques- 
tion. Jupiter,  again,  is  a  planet,  and  though  he  is  so  much 
larger  than  the  earth  that  we  might  be  disposed  at  the  out- 
set to  regard  him  as  a  body  of  another  order,  we  must  be 
so  guided  by  analogies  (which,  after  all,  may  be  imaginary) 
as  to  consider  that  his  size  only  renders  him  so  much  the 
nobler  an  abode  for  such  life  as  we  are  familiar  with:  and 
instead  of  being  struck  by  the  fact  that  Jupiter,  unlike 
Mars,  shows  no  polar  snow-caps,  we  are  to  direct  our  atten- 
tion to  his  belts,  and  to  regard  them  as  cloud-belts  analo- 
gous to  the  tropical  cloud-zone  of  the  earth.  Nor  are 
we  to  inquire  too  closely  whether  the  aspect  of  his  equa- 
torial belt,  to  say  nothing  of  his  other  belts,  corresponds 
in  any  degree  with  that  which  the  cloud  zone  of  our  earth 
would  present  to  observers  on  another  planet.  Let  it  suffice 
to  note  a  few  analogies,  as  thus:  "The  earth  is  a  planet, 
Jupiter  is  a  planet;  the  earth  rotates  and  therefore  has  a 
day,  Jupiter  rotates  and  has  a  day;  the  earth  has  a  year, 
Jupiter  has  a  year;  the  earth  has  clouds,  Jupiter  has  clouds; 
the  earth  has  a  moon,  Jupiter  has  four  moons;  this  done, 
every  other  consideration  may  be  conveniently  overlooked, 
and  we  may  proceed  to  descant  on  the  wonderful  extent 
and  dignity  of  this  distant  world,  with  as  little  question  of 
its  being  inhabited  as  though  we  had  seen  with  our  own 


A   NEW   THEORY    OF  LIFE   IN  OTHER  WORLDS     87 

eyes  the  creatures  which  exist  upon  the  planet's  surface. 
So  with  Saturn,  and  the  rest." 

Whewell  broke  through  all  these  old-fashioned  meth- 
ods. He  dealt  with  the  several  planets  on  the  true  scientific 
principle  long  since  enunciated  by  Descartes,  taking  noth- 
ing for  granted  that  had  not  been  proved.  He  showed  how 
unlike  the  conditions  prevailing  in  the  other  planets  must 
be  to  those  existing  on  the  earth,  and  without  pretending 
to  demonstrate  absolutely  that  none  of  the  higher  forms  of 
life  can  exist  on  certain  planets,  he  showed  that  at  any  rate 
the  probabilities  are  in  favour  of  that  hypothesis.  Passing 
on  to  the  stars,  he  did  good  service  by  showing  how  much 
had  been  taken  for  granted  by  astronomers  in  their  assump- 
tions respecting  those  orbs;  nor  is  the  value  of  his  work, 
in  this  field,  by  any  means  diminished  by  the  circumstance 
that  during  recent  years  evidence  which  was  wanting  when 
Whewell  wrote  has  been  obtained,  and  the  stars  have  been 
shown  demonstratively  to  be  suns.  And  lastly,  he  dealt 
in  an  independent  and  therefore  instructive  manner  with 
the  star-cloudlets  or  nebulae,  giving  many  strong  reasons 
for  doubting  the  views  which  were  at  that  time  repeated  in 
every  text-book  of  astronomy. 

The  conclusions  to  which  Whewell  was  led  were:  (i) 
that  no  sufficient  reason  exists  for  believing  in  other  worlds 
than  ours;  and  (2)  if  the  other  planets  are  inhabited,  it 
can  only  be,  in  all  probability,  by  creatures  belonging  to 
the  lowest  orders  of  animated  existence.  He  somewhat 
softened  the  harshness  of  these  inferences  by  pointing  out 
that  our  conceptions  of  the  glories  of  God's  kingdom  need 
not  be  enfeebled  by  our  doubts  as  to  the  existence  of  life 
in  the  planets  of  our  own  system,  or  of  systems  circling 
around  other  suns.  "  However  destitute,"  he  wrote, 
"  planets,  moon,  and  rings  may  be  of  inhabitants,  they  are 
at  least  vast  scenes  of  God's  presence,  and  of  the  activity 
with  which  he  carries  into  effect  everywhere  the  laws  of 
Nature;  and  the  glory  of  creation  arises  from  its  being,  not 
only  the  product  but  the  constant  field  of  God's  activity 
and  thought,  wisdom,  and  power."  And,  in  passing,  I 


88  PROCTOR 

may  note  that  Sir  David  Brewster,  when  commenting 
somewhat  angrily  and  contemptuously  on  this  remark, 
failed  really  to  grasp  Whewell's  meaning.  Brewster  was 
at  great  pains  to  show  how  large  a  portion  of  the  glories  of 
the  heavens  is  invisible  and  useless  to  man;  but  Whewell 
was  manifestly  not  referring  to  the  glories  of  God  as  re- 
vealed to  man,  but  as  they  exist  in  themselves.  It  must 
be  admitted,  indeed,  even  by  those  who  prefer  Brewster's 
theory,  that  he  maintained  it  with  much  more  warmth  than 
was  necessary  in  such  a  discussion.  In  presence  of  Whew- 
ell's philosophic,  calm,  and  dispassionate  force  of  rea- 
soning, there  was  something  almost  ludicrous  in  the  im- 
passioned outbursts  of  the  great  physicist  who  took  the 
doctrine  of  life  in  other  worlds  under  his  protection. 
"  Where,"  says  he,  "  is  the  grandeur,  where  the  utility, 
where  the  beauty,  where  the  poetry,  of  the  two  almost  in- 
visible stars  which  usurp  the  celestial  names  of  Uranus  and 
Neptune,  and  which  have  been  seen  by  none  but  a  very 
few  even  of  the  cultivators  of  astronomy?  The  seaman  in 
the  trackless  ocean  never  seeks  their  guidance;  to  him  they 
have  not  even  the  value  of  the  Pole  Star;  they  contribute 
nothing  to  the  arts  of  terrestrial  life:  they  neither  light  the 
traveller  on  his  journey,  nor  mark  by  their  feeble  ray  the 
happy  hours  which  are  consecrated  to  friendship  and  to 
love."  All  this  is  very  pretty  writing,  but  it  is  very  little 
to  the  purpose,  and  while  it  has  no  bearing  whatever  on 
what  Whewell  had  urged,  it  is  a  very  long  way  from  estab- 
lishing what  Brewster  desired  to  prove — viz.,  that  "  Uranus 
and  Neptune  must  have  been  created  for  other  and  nobler 
ends;  to  be  the  abodes  of  life  and  intelligence,  the  colossal 
temples  where  their  Creator  is  recognised  and  worshipped; 
the  remotest  watch-towers  of  our  system,  from  which  his 
works  may  be  better  studied,  and  his  distant  glories  more 
readily  described." 

Here,  however,  are  two  theories — opposed  to  each 
other,  and  not  admitting  of  being  reconciled.  If  we  are 
to  make  a  selection  between  them,  to  which  shall  we  turn 
in  preference?  The  balance  of  evidence  is  on  the  whole  in 


A   NEW   THEORY   OF   LIFE   IN   OTHER   WORLDS      89 

favour  of  Whewell's  (so  at  least  the  matter  presents  itself 
to  me  after  careful  and  long-continued  study);  but  certainly 
Brewster's  is  the  theory  which  commends  itself  most  fa- 
vourably to  the  mind  which  would  believe  that  God  "  hath 
done  all  things  well,"  and  that  nothing  that  he  has  made 
was  made  in  vain.  Even  those  who,  like  myself,  are  indis- 
posed to  admit  that  the  ways  and  works  of  God  are  to  be 
judged  by  our  conceptions  of  the  fitness  of  things  (though 
we  may  be  altogether  certain  that  all  things  are  made  in 
wisdom  and  fitness),  would  prefer  to  accept  the  Brewsterian 
theory,  if  decision  were  to  be  made  between  the  two.  For, 
what  amount  of  evidence  could  reconcile  us  to  the  belief 
(even  though  it  forced  this  belief  upon  us)  that  our  earth 
alone  of  all  the  countless  orbs  which  people  space,  is  the 
abode  of  reasoning  creatures,  capable  of  recognising  the 
glories  of  the  universe,  and  of  lauding  the  Creator  of  those 
wonders  and  of  their  own  selves?  Nevertheless  we  must  be 
guided  in  these  matters  by  evidence,  not  by  sentiment 
— by  facts,  not  by  our  feelings.  It  is  well,  therefore,  to 
note  that  the  decision  does  not  lie  between  the  two  theories 
which  have  just  been  dealt  with.  Another  theory,  holding 
a  position  intermediate  between  those  two,  and  combining 
in  my  judgment  the  evidence  which  favours  one  theory 
with  the  fitness  characterizing  the  other,  remains  yet  to 
be  presented. 

I  propose  to  take,  as  the  basis  of  the  new  theory  of  life 
in  other  worlds,  the  analogy  which  has  commonly  been 
regarded  as  affording  the  strongest  evidence  in  favour  of 
the  Brewsterian  theory— only  I  shall  take  a  more  extended 
view  of  the  subject  than  has  been  customary. 

Before  introducing  that  Brewsterian  argument,  I  may 
remark  that  the  mere  fact  that  our  earth  is  an  inhabited 
world  is  not  in  itself  sufficient  even  to  render  probable  the 
theory  that  there  is  life  in  other  worlds  than  ours.  An 
equally  strong  argument  might  be  derived  against  that 
theory  from  the  study  of  our  moon — the  only  other  planet 
of  which  we  have  obtained  reliable  information — for  few 
can  suppose  that  the  moon  is  fit  to  be  the  abode  of  life. 


90  PROCTOR 

Since,  then,  of  the  two  planets  we  can  examine,  one — the 
earth — is  inhabited,  while  the  other — the  moon — is  proba- 
bly not  inhabited,  the  only  evidence  we  have  is  almost 
equally  divided  between  the  Whewellite  and  Brewsterian 
theories,  whatever  balance  remains  in  favour  of  the  latter 
being  too  slight  to  afford  any  sufficient  basis  for  a  con- 
clusion. 

But  while  this  reasoning  is  just,  as  applied  to  the  mere 
fact  that  the  earth  is  inhabited,  it  is  by  no  means  capable 
of  overthrowing  the  evidence  which  is  derived  from  the 
manner  in  which  life  exists  on  the  earth.  When  we  con- 
sider the  various  conditions  under  which  life  is  found  to 
prevail,  that  no  difference  of  climatic  relations  6r  of  eleva- 
tion, of  land  or  of  air  or  of  water,  of  soil  in  land,  of  fresh- 
ness or  saltness  in  water,  of  density  in  air,  appears  (so  far 
as  our  researches  have  extended)  to  render  life  impossible, 
we  are  compelled  to  infer  that  the  power  of  supporting  life 
is  a  quality  which  has  an  exceedingly  wide  range  in  Nature. 
I  refrain,  it  will  be  noticed,  from  using  here  the  usual  ex- 
pression, and  saying,  as  of  yore,  that  "  the  great  end  and 
aim  of  all  the  workings  of  Nature  is  to  afford  scope  and 
room  for  the  support  of  life,"  because  this  mode  of  speak- 
ing may  be  misunderstood.  We  can  see  what  Nature 
actually  does,  and  we  may  infer,  if  we  so  please,  that 
such  or  such  is  the  end  and  aim  of  the  God  of  Nature; 
nevertheless  we  must  remember  that  the  evidence 
we  have  belongs  to  the  former  relation,  not  to  the 
latter.  I  am  careful  to  dwell  on  this  point  because  the 
longer  I  study  such  matters  the  more  clearly  do  I  rec- 
ognise the  necessity  of  most  studiously  limiting  our  state- 
ments to  that  which  the  evidence  before  us  really  estab- 
lishes. 

Passing  beyond  the  evidence  which  the  earth  at  present 
affords,  we  find  that  during  many  ages  the  earth  has  pre- 
sented a  similar  scene.  "  Geology,"  I  wrote  four  years 
ago,  "  teaches  us  of  days  when  this  earth  was  peopled  with 
strange  creatures  such  as  now  are  not  found  upon  its  sur- 
face. We  turn  our  thoughts  to  the  epochs  when  these 


A   NEW  THEORY  OF   LIFE   IN   OTHER  WORLDS      91 

monsters  throve  and  multiplied,  and  picture  to  ourselves 
the  appearance  which  our  earth  then  presented.  Strange 
forms  of  vegetation  clothe  the  scene  which  the  mind's  eye 
dwells  upon.  The  air  is  heavily  laden  with  moisture  to 
nourish  the  abundant  flora;  hideous  reptiles  crawl  over 
their  slimy  domain,  battling  with  each  other,  or  with  the 
denizens  of  the  forest;  huge  batlike  creatures  sweep 
through  the  dusky  twilight  which  constituted  the  primeval 
day;  wreird  monsters  pursue  their  prey  amid  the  depths  of 
ocean:  and  we  forget,  as  we  dwell  upon  the  strange  forms 
which  existed  in  those  long-past  ages,  that  the  scene  now 
presented  by  the  earth  is  no  less  wonderful,  and  that  the 
records  of  our  time  may,  perhaps,  seem  one  day  as  per- 
plexing as  we  now  find  those  of  the  geological  eras."  In 
the  past,  then,  as  in  the  present,  this  earth  was  inhabited 
by  countless  millions  of  living  creatures,  and  during  the 
enormous  period  which  has  elapsed  since  life  first  appeared 
on  the  surface  of  the  earth,  myriads,  if  not  millions,  of  orders 
of  living  creatures  have  appeared,  have  lived  the  life  ap- 
pointed to  their  order,  and  have  vanished,  or  exist  only 
under  modified  forms.  As  each  individual  has  had  its 
period  of  life,  so  also  has  each  race,  and  we  may  say  with 
the  poet  (noting  always  that  the  personification  of  Nature 
is  but  a  poetical  idea,  and  does  not  present  any  real  sub- 
stantive truth) : 

"  Are  God  and  Nature  then  at  strife, 
That  Nature  lends  such  evil  dreams? 
So  careful  of  the  type  she  seems, 
So  careless  of  the  single  life. 


"  '  So  careful  of  the  type?  '  but  no, 

From  scarped  cliff  and  quarried  stone 
She  cries,  '  A  thousand  types  are  gone ; 
I  care  for  nothing,  all  shall  go.'  " 

Abundant  life,  in  ever-varying  forms,  and  under  all- 
various  conditions,  continuing  age  after  age  during  hun- 
dreds of  thousands  of  years,  such  is  what  our  earth  presents 
to  us  when  we  turn  our  thoughts  to  its  past  history.  And 
looking  forward,  a  similar  scene  is  presented  to  our  con- 


92 


PROCTOR 


templation.  For  many  a  long  century,  probably  for  hun- 
dreds of  thousands  of  years,  life  will  continue  on  the  earth, 
unless  some  catastrophe  (the  occurrence  of  which  we  have 
as  yet  no  reason  to  anticipate)  should  destroy  life  suddenly 
from  off  her  surface. 

So  viewing  this  earth,  we  seem  to  find  forced  upon  us 
the  belief  that  the  support  of  life  is  the  object  for  which 
the  earth  was  created,  and  thus  we  are  led  to  regard  the 
other  orbs  which,  like  her,  circle  around  a  central  sun,  as 
intended  to  be  the  abode  of  life.  The  only  object  which, 
so  far  as  we  can  see,  the  earth  has  fulfilled  during  an  in- 
definitely long  period  has  been  to  present  a  field,  so  to 
speak,  for  the  support  of  life,  nor  can  we  recognise  any 
other  purpose  which  she  will  fulfil  in  the  future.  If  we 
admit  this,  and  if  we  also  believe  that  God  made  nothing 
without  some  purpose,  of  course  we  have  no  choice  but 
to  admit  that  the  purpose  with  which  the  earth  was  made 
was  the  support  of  life.  And  reasoning  from  analogy,  we 
infer  that  the  other  planets,  as  well  those  of  our  own  sys- 
tem as  those  which  we  believe  to  exist,  "  wheeling  in  per- 
petual round,"  as  attendants  upon  other  suns,  were  simi- 
larly created  to  be  the  abode  of  life.3 

*  I  shall  venture  to  quote  here  the  once  celebrated  argument  ad- 
vanced by  Dr.  Bentley  in  favour  of  the  plurality  of  worlds:  "  Consid- 
ering," he  says,  "  that  the  soul  of  one  virtuous  and  religious  man  is 
of  greater  worth  and  excellency  than  the  sun  and  all  his  planets,  and  all 
the  stars  in  the  heavens,  their  usefulness  to  man  might  be  the  sole  end 
of  their  creation  if  it  could  be  proved  that  they  were  as  beneficial  to 
us  as  the  Pole  Star  formerly  was  for  navigation,  or  as  the  moon  is  for 
producing  the  tides  and  lighting  us  on  winter  nights.  But  we  dare  not 
undertake  to  show  what  advantage  is  brought  to  us  by  those  innumerable 
stars  in  the  galaxy  of  other  parts  of  the  firmament,  not  discernible  by 
naked  eyes,  and  yet  each  many  thousand  times  bigger  than  the  whole 
body  of  the  earth.  If  you  say  they  beget  in  us  a  great  idea  and 
veneration  of  the  mighty  Author  and  Governor  of  such  stupendous 
bodies,  and  excite  and  devote  our  minds  to  his  adoration  and  praise, 
you  say  very  truly  and  well.  But  would  it  not  raise  in  us  a  higher 
apprehension  of  the  infinite  majesty  and  boundless  beneficence  of  God, 
to  suppose  that  those  remote  and  vast  bodies  were  formed,  not  merely 
upon  our  account,  to  be  peeped  at  through  an  optic  glass,  but  for  dif- 
ferent ends  and  nobler  purposes?  And  yet  who  will  deny  that  there 
are  great  multitudes  of  lucid  stars  even  beyond  the  reach  of  the  best 


A  NEW  THEORY  OF  LIFE   IN  OTHER  WORLDS      93 

But,  before  we  infer  from  the  strength  of  this  reason- 
ing that  the  other  planets  are  inhabited  worlds,  let  us  look 
somewhat  more  closely  into  the  circumstances,  or  rather, 
instead  of  examining  only  a  portion  of  the  evidence,  let 
us  take  a  wider  survey  and  examine  all  the  evidence  we 
possess.  It  may  appear,  at  a  first  view,  that  already  we 
are  dealing  with  periods  which,  to  our  conceptions,  are 
practically  infinite.  How  long,  compared  with  the  brief 
span  of  human  life,  are  the  eras  with  which  history  deals! 
how  enormous,  even  by  comparison  with  these  eras,  appears 
the  range  of  time  (tens  of  thousands,  if  not  hundreds  of 
thousands,  of  years)  since  man  first  appeared  upon  this 
earth!  and,  according  to  the  teachings  of  geology,  we 
have  to  deal  with  a  yet  higher  order  of  time  in  passing 
to  the  beginning  of  life  upon  our  globe.  From  one 
million  of  years  to  ten  millions!  It  is  between  such  lim- 
its, say  the  most  experienced  geologists,  that  the  choice 
lies.  Surely  we  may  be  content  with  periods  such  as 
these,  periods  as  utterly  beyond  our  powers  of  conception 
as  the  duration  of  the  Pyramids  would  be  to  creatures 
like  the  ephemeron,  did  such  creatures  possess  the  power 
of  reason! 

And  yet,  why  should  we  stop  at  the  beginning  of  life 
upon  this  earth?  We  have  passed  to  higher  and  higher 
orders  of  time-intervals,  but  the  series  has  no  limit  that 
we  know  of,  while  it  possesses  terms,  recognisable  by  us, 

telescopes ;  and  that  every  visible  star  may  have  opaque  planets  revolv- 
ing about  them  which  we  can  not  discover?  Now,  if  they  were  not 
created  for  our  sakes  it  is  certain  and  evident  that  they  were  not  made 
for  their  own;  for  matter  has  no  life  or  perception,  is  not  conscious 
of  its  own  existence,  nor  capable  of  happiness,  nor  gives  the  sacrifice  of 
praise  and  worship  to  the  author  of  its  being.  It  remains,  therefore, 
that  all  bodies  were  formed  for  the  sake  of  intelligent  minds;  and  as 
the  earth  was  principally  designed  for  the  being  and  service  and  con- 
templation of  men,  why  may  not  all  other  planets  be  created  for  the 
like  uses,  each  for  its  own  inhabitants  which  have  life  and  under- 
standing? "  The  objection  to  Dr.  Bentley's  argument  resides,  not  in 
the  belief  which  he  expresses  in  the  wisdom  and  beneficence  of  the 
Creator,  but  in  the  confidence  with  which  he  assumes  that  the  Creator 
had  such  and  such  purposes — and  not  perhaps  others  such  as  we  not 
only  can  not  discover,  but  can  not  even  conceive. 


94 


PROCTOR 


of  higher  order  than  those  we  have  been  dealing  with.  We 
know  that  in  the  far-off  times  before  life  appeared, 

"  The  solid  Earth  whereon  we  tread 
In  tracts  of  fluent  heat  began, 
And  grew  to  seeming-random  forms, 
The  seeming  prey  of  cyclic  storms." 

Let  us  look  back  at  that  part  of  the  earth's  history,  and 
see  whether  the  long  periods  which  we  have  contemplated 
may  not  be  matched  and  more  than  matched  by  the  aeons 
which  preceded  them.  When  we  thus 

"  Contemplate  all  this  work  of  Time 
The  giant  labouring  in  his  youth," 

we  see  how  far  we  have  been  from  recognising  the  true 
breadth  of  the  mighty  waves  on  one  of  which  the  life  upon 
this  earth  has  been  borne,  we  see  that  as  yet  we  have  not 

"  Come  on  that  which  is,  and  caught 
The  deep  pulsations  of  the  world — 
^Eonian  music  measuring  out 
The  steps  of  time." 

Taking  as  the  extremest  span  of  the  past  existence  of  life 
upon  the  earth  ten  millions  of  years,  we  learn  from  the  re- 
searches of  physicists  that  the  age  preceding  that  of  life 
(the  age  during  which  the  world  was  a  mass  of  molten 
rock)  lasted  more  than  thirty-five  times  as  long,  since 
Bischof  has  shown  that  the  earth  would  require  three  hun- 
dred and  fifty  millions  of  years  to  cool  down  from  a  tem- 
perature of  2,000°  C.  to  200°  C.  But  far  back  beyond  the 
commencement  of  that  vast  era,  our  earth  existed  as  a 
nebulous  mass,  nor  can  we  form  even  a  conjecture  as  yet 
respecting  the  length  of  time  during  which  that  earlier 
stage  of  the  earth's  existence  continued. 

So  much  for  the  past.  Of  the  future  we  know  less. 
But  still  we  recognise,  not  indistinctly,  a  time  when  all  life 
will  have  ceased  upon  the  earth.  Whether  by  the  process 
of  refrigeration  which  is  going  on,  or  by  the  gradual  ex- 
haustion of  the  forces  which  at  present  reside  in  the  earth, 


A   NEW  THEORY  OF   LIFE  IN   OTHER  WORLDS      95 

or  by  the  change  in  the  length  of  the  day  which  we  know 
to  be  slowly  taking  place,  a  time  must  come  when  the  con- 
dition of  our  earth  will  no  longer  be  suited  for  the  support 
of  life.  Or  it  may  be  that  Stanilas  Meunier  is  right  in 
his  theory  that  as  a  planet  grows  older,  the  oceans,  and 
even  the  atmosphere,  are  gradually  withdrawn  into  the  in- 
terior of  the  planet's  globe,  where  space  is  formed  for  them 
by  the  cooling  and  contracting  of  the  solid  frame  of  the 
planet.  But  apart  from  all  such  considerations,  we  know 
that  a  process  of  exhaustion  is  taking  place,  even  in  the 
sun  himself,  whence  all  that  exists  upon  the  earth  derives 
its  life  and  daily  nourishment.  So  that  indirectly  by  the 
dying  out  of  the  source  of  life,  if  not  directly  by  the  dying 
out  of  life,  this  earth  must  one  day  become  as  bleak  and 
desolate  a  scene  as  we  believe  the  moon  to  be  at  this 
present  time. 

It  is  easy  to  recognise  the  bearing  of  these  considera- 
tions upon  the  question  of  life  in  other  worlds.  We  had 
been  led,  by  the  contemplation  of  the  long  continuance  of 
life  upon  this  earth,  to  regard  the  support  of  life  as  in 
a  sense  the  object  of  planetary  existence,  and  therefore  to 
view  the  other  planets  as  the  abode  of  life.  But  we  now 
see  that  the  time  during  which  life  has  existed  on  the  earth 
has  been  a  mere  wavelet  in  the  sea  of  our  earth's  lifetime, 
this  sea  itself  being  but  a  minute  portion  of  the  infinite 
ocean  of  time,  while,  as  Tyndall  has  well  remarked,  in  that 
infinite  ocean,  the  history  of  man  (the  sole  creature  known 
to  us  that  can  appreciate  the  wonders  of  creation)  is  but 
the  merest  ripple.  We  learn,  then,  from  the  earth's  his- 
tory, a  lesson  the  very  reverse  of  that  which  before  we  had 
seemed  so  clearly  to  read  there.  It  is  not  the  chief,  but 
only  a  minute  portion  of  the  earth's  existence  which  has 
been  characterized  by  the  existence  of  life  upon  our  globe; 
and  if  we  adopted  the  teaching  now  brought  before  us,  as 
readily  as  before  we  learned  that  other  lesson,  we  should 
say,  "  It  is  not  the  chief,  but  only  an  utterly  subordinate 
part  of  Nature's  purpose,  to  provide  for  the  existence  and 
support  of  life." 


96  PROCTOR 

We  have  been  led  by  the  study  of  the  probable  past 
history  of  the  earth,  and  by  the  consideration  of  her  prob- 
able future  fortunes,  to  the  conclusion  that  although  life 
has  existed  on  her  surface  for  an  enormously  long  period, 
and  will  continue  for  a  corresponding  period  in  the  future, 
yet  the  whole  duration  of  life  must  be  regarded  but  as  a 
wave  on  the  vast  ocean  of  time,  while  the  duration  of  the 
life  of  creatures  capable  of  reasoning  upon  the  wonders 
which  surround  them,  is  but  as  a  ripple  upon  the  surface 
of  such  a  wave.  It  matters  little  then  whether  we  take  life 
itself,  without  distinction  of  kind  or  order,  or  whether  we 
take  only  the  life  of  man,  we  still  find  a  disproportion  which 
must  be  regarded  as  practically  infinite,  between  the  dura- 
tion of  such  life,  and  the  duration  of  the  preceding  and 
following  periods  when  there  has  been  and  will  be  no  such 
life  upon  the  earth. 

But  yet,  in  passing,  I  can  not  but  point  to  the  fact  that 
in  considering  the  usual  arguments  of  life  in  other  worlds, 
I  might  limit  myself  to  the  existence  of  rational  beings. 
It  would  be  difficult  to  show  that  mere  life,  without  the 
power  which  man  possesses  of  appreciating  the  wonders 
of  the  universe,  is  a  more  fitting  final  purpose  in  creation 
than  the  existence  of  lifeless  but  moving  masses  like  the 
suns  and  their  attendant  planets.  The  insect  or  the  fish, 
the  bird  or  the  mammal,  the  minutest  miscroscopic  animal- 
cule or  the  mightiest  cetacean,  may  afford  suggestive  indica- 
tions of  what  we  describe  as  beneficent  contrivance;  yet  it 
is  hard  to  see  in  what  essential  respect  a  universe  of  worlds 
beyond  our  own,  inhabited  only  by  such  animals,  would 
accord  better  writh  those  ideas  which  the  believers  in  the 
plurality  of  worlds  entertain  respecting  the  purpose  of  the 
Almighty,  than  a  universe  with  none  but  vegetable  life,  or 
a  universe  with  no  life  at  all,  yet  replete  with  wonderful  and 
wonderfully  moving  masses  of  matter.  It  is  rational  life 
alone  to  which  the  arguments  of  our  Brewsters  and  Chal- 
mers really  relate.  Nor  would  it  be  difficult  to  raise  here 
another  perplexing  consideration,  by  inquiring  what  de- 
gree of  cultivation  of  the  intellect  in  human  races  accords 


A   NEW   THEORY   OF   LIFE   IN   OTHER  WORLDS      97 

with  the  "  argument  from  admiration  "  which  the  followers 
of  Brewster  delight  to  employ.  The  savage  engaged  in 
the  mere  effort  to  support  life  or  to  combat  his  foes,  knows 
nothing  of  the  glories  whereof  science  tells  us.  The  won- 
ders of  Nature,  so  far  as  they  affect  him  at  all,  tend  to  give 
ignoble  and  debasing  ideas  of  the  being  or  beings  to  whose 
power  he  attributes  the  occurrence  of  natural  phenomena. 
Nor,  as  we  advance  in  the  scale  of  civilization,  do  we 
quickly  arrive  at  the  stage  where  the  admiration  of  Nature 
begins  to  be  an  ordinary  exercise  even  of  a  few  minds. 
Still  less  do  we  arrive  quickly,  even  in  reviewing  the 
progress  of  the  most  civilized  races,  at  the  stage  when  the 
generality  of  men  give  much  of  their  thoughts  to  the 
natural  wonders  which  surround  them.  Is  it  saying  too 
much  to  assert  that  this  stage  has  never  yet  been  attained 
by  any  nation,  even  the  most  advanced  and  the  most  cul- 
tured? If  we  limit  ourselves,  however,  to  the  existence 
merely  of  some  few  nations,  among  whom  the  study  of 
Nature  has  been  more  or  less  in  vogue,  how  brief  in  the 
history  of  this  earth  has  been  the  period  when  such  nations 
have  existed!  how  brief  the  continuance  of  those  among 
such  nations  which  belong  to  the  past,  and  whose  whole 
history  is  thus  known  to  us!  how  few  even  in  such  nations 
the  men  who  have  been  so  deeply  impressed  with  the  won- 
ders of  Nature  as  to  be  led  to  the  utterance  of  their 
thoughts!  If  the  life  of  man  is  but  as  a  ripple  where  life 
itself  is  as  a  wave  on  the  ocean  of  time,  surely  the  life 
of  man  as  the  student  and  admirer  of  Nature  is  but  as  the 
tiniest  of  wave-crests  upon  the  ripple  of  human  life. 

How,  then,  does  all  this  bear  upon  the  question  of  life 
in  other  worlds?  The  answer  will  be  manifest  if  we  apply 
to  these  considerations  the  same  argument  which  Brewster 
and  Chalmers  have  applied  to  the  evidence  which  indicates 
the  enormous  duration  of  life  upon  the  earth.  Since  this 
enormous  duration,  taking  life  even  in  its  most  general 
aspect,  has  been  shown  to  be  as  a  mere  nothing  by  com- 
parison with  the  practically  infinite  duration  of  the  earth 
without  life,  the  argument  as  respects  life  in  any  other 
7 


98  PROCTOR 

world  (at  least,  in  any  world  of  which  antecedently  we  know 
nothing)  must  be  directly  reversed.  It  is  far  more  probable 
that  that  world  is  now  passing  through  a  part  of  the  stage 
preceding  the  appearance  of  life,  or  of  the  stage  following 
the  appearance  of  life,  than  that  this  particular  epoch  be- 
longs to  the  period  when  that  particular  world  is  inhabited. 
If,  indeed,  we  had  some  special  reason  for  believing  that 
this  epoch  to  which  terrestrial  life  belongs  has  some  special 
importance  as  respects  the  whole  universe,  we  might  feel 
unwilling  to  consider  the  question  of  life  in  any  other  world 
independently  of  preconceptions  derived  from  our  experi- 
ence in  this  world.  But  I  apprehend  that  we  have  no  rea- 
son whatever  for  so  believing.  It  appears  to  me  that  such 
a  belief — that  is,  the  belief  that  life  in  this  earth  corre- 
sponds with  a  period  special  for  the  universe  itself — is  as 
monstrous  as  the  old  belief  that  our  earth  is  the  centre  of 
the  universe.  It  is,  in  fact,  a  belief  which  bears  precisely  the 
same  relation  to  time  that  the  last-mentioned  belief  bears 
to  space.  According  to  one  belief,  the  minute  space  occu- 
pied by  our  earth  was  regarded  as  the  central  and  most 
important  part  of  all  space,  and  the  only  part  which  the 
Creator  had  specially  in  his  plans,  so  to  speak,  in  creating 
the  universe;  according  to  the  other,  the  minute  time 
occupied  by  the  existence  of  life  on  the  earth  is  the  central 
and  most  important  part  of  all  time,  and  the  only  part 
during  which  the  Creator  intended  that  living  creatures 
should  exist  anywhere.  Both  ideas  are  equally  untenable, 
though  one  only  has  been  formally  discarded. 

This  present  time,  then,  is  a  random  selection,  so  to 
speak,  regarded  with  reference  to  the  existence  of  life  in 
any  other  world,  and  being  a  random  selection,  it  is  much 
more  likely  to  belong  to  the  period  when  there  is  no  life 
there.  Let  me  illustrate  my  meaning  by  an  example.  Sup- 
pose I  know  that  a  friend  of  mine,  living  at  a  distance,  will 
be  at  home  for  six  minutes  exactly,  some  time  between 
noon  and  ten  on  any  given  day,  but  that  I  have  no  means 
of  forming  any  opinion  as  to  when  the  six  minutes  will  be. 
Then,  if  at  any  given  moment,  say  at  three,  I  ask  myself 


A  NEW  THEORY  OF  LIFE   IN   OTHER  WORLDS      99 

the  question,  "  Is  my  friend  at  home?  "  although  I  can  not 
know,  I  can  form  an  opinion  as  to  the  probability  of  his 
being  so.  There  are  six  hundred  minutes  between  noon 
and  ten,  and  he  is  to  be  at  home  only  six  minutes,  or  the 
one-hundredth  part  of  the  time;  accordingly,  the  chance 
that  he  is  at  home  is  one  in  a  hundred,  or  speaking  in  a 
general  way  it  is  much  more  likely  that  he  is  not  at  home 
than  that  he  is.  And  so  precisely  with  any  given  planet, 
apart  from  any  evidence  we  may  have  as  to  its  condition — 
what  we  know  about  life  on  our  earth  teaches  us  that  the 
probability  is  exceedingly  minute  that  that  planet  is  in- 
habited. The  argument  is  the  favourite  argument  from 
analogy.  Thus:  life  on  our  earth  lasts  but  a  very  short 
time  compared  with  the  duration  of  the  earth's  existence; 
therefore  life  in  any  given  planet  lasts  but  a  very  short  time 
compared  with  the  planet's  existence;  accordingly,  the 
probability  that  that  planet  is  inhabited  at  this  present  mo- 
ment of  time  is  exceedingly  small,  being,  in  fact,  as  the 
number  of  years  of  life  to  the  number  of  years  without  life, 
or  as  one  chance  in  many  hundreds  at  the  least. 

This  applies  to  the  planets  of  our  solar  system  only  in 
so  far  as  we  are  ignorant  of  their  condition.  We  may  know 
enough  about  some  of  them  to  infer  either  a  much  higher 
probability  that  life  exists,  or  almost  certainly  that  life  can 
not  exist.  Thus  we  may  view  the  condition  of  Venus  or 
Mars  as  perchance  not  differing  so  greatly  from  that  of 
our  earth  as  to  preclude  the  probability  that  many  forms 
of  life  may  exist  on  those  planets.  Or,  on  the  other  hand, 
we  may  believe  from  what  we  know  about  Jupiter  and 
Saturn  that  both  these  planets  are  still  passing  through  the 
fiery  stages  which  belong  to  the  youth  of  planet  life;  while 
in  our  moon  we  may  see  a  world  long  since  decrepit,  and 
now  utterly  unfit  to  support  any  forms  of  animated  exist- 
ence. But  even  in  the  case  of  our  solar  system,  though  the 
evidence  in  some  cases  against  the  possibility  of  life  is  ex- 
ceedingly strong,  we  do  not  meet  with  a  single  instance 
in  which  evidence  of  the  contrary  kind  is  forcible,  still  less 
decisive.  So  that  in  the  solar  system  the  evidence  is  almost 


100  PROCTOR 

as  clear  in  favour  of  the  conclusion  above  indicated  as  where 
we  reason  about  worlds  of  whose  actual  condition  we  know 
nothing.  As  respects  such  worlds — that  is,  as  respects  the 
members  of  those  systems  of  worlds  which  circle,  as  we 
believe  (from  analogy),  around  other  suns  than  ours — the 
probability  that  any  particular  world  is  inhabited  at  this 
present  time  is  exceedingly  small. 

But  let  us  next  consider  what  is  the  probability  that 
there  is  life  on  some  member  or  other  of  a  scheme  of  worlds 
circling  around  any  given  sun.  Here,  again,  the  argument 
is  from  analogy,  being  derived  from  what  we  have  learned 
or  consider  probable  in  the  case  of  our  own  system.  And  I 
think  we  may  adopt  as  probable  some  such  view  as  I  shall 
now  present.  Each  planet,  according  to  its  dimensions, 
has  a  certain  length  of  planetary  life,  the  youth  and  age 
of  which  include  the  following  eras:  a  sunlike  state;  a  state 
like  that  of  Jupiter  or  Saturn,  when  much  heat  but  little 
light  is  evolved;  a  condition  like  that  of  our  earth;  and 
lastly,  the  stage  through  which  our  moon  is  passing,  which 
may  be  regarded  as  planetary  decrepitude.  In  each  case  of 
world  existences  the  various  stages  may  be  longer  or 
shorter,  as  the  whole  existence  is  longer  or  shorter,  so  that, 
speaking  generally,  the  period  of  habitability  bears  the  same 
proportion  in  each  world  to  the  whole  period  of  its  exist- 
ence; or  perhaps  there  is  no  such  uniform  proportion, 
while,  nevertheless,  there  exists  in  all  cases  that  enormous 
excess  of  the  period  when  no  life  is  possible  over  the  period 
of  habitability.  In  either  case,  it  is  manifest  that  regarding 
the  system  as  a  whole,  now  one,  now  another  planet  (or 
more  generally,  now  one,  now  another  member  of  the  sys- 
tem) would  be  the  abode  of  life,  the  smaller  and  shorter- 
lived  having  their  turn  first,  then  larger  and  larger  mem- 
bers, until  life  has  existed  on  the  mightiest  of  the  planets, 
and  even  at  length  upon  the  central  sun  himself.  We  need 
not  concern  ourselves  specially  with  the  peculiarities  affect- 
ing the  succession  of  life  in  the  case  of  subordinate  systems, 
or  of  the  members  of  the  asteroidal  family,  or  in  other  cases 
where  we  have  little  real  knowledge  to  guide  us:  the  gen- 


A   NEW  THEORY  OF   LIFE   IN   OTHER  WORLDS    ioi 

eral  conclusion  remains  the  same,  that  life  would  appear 
successively  in  planet  after  planet,  step  by  step  from  the 
smaller  to  the  larger,  until  the  approach  of  the  last  scene 
of  all,  when  life  would  have  passed  from  all  the  planets,  and 
our  sun  would  alone  remain  to  be  in  due  time  inhabited, 
and  then  in  turn  to  pass  (by  time  intervals  to  us  practically 
infinite)  to  decrepitude  and  death. 

During  all  this  progression,  the  intervals  without  life 
would  in  all  probability  be  far  longer  than  those  when  one 
or  other  planet  was  inhabited.  In  fact,  the  enormous  ex- 
cess of  the  lifeless  periods  for  our  earth  over  the  period 
of  habitability  renders  the  conclusion  all  but  certain  that 
the  lifeless  gaps  in  the  history  of  the  solar  system  must  last 
very  much  longer  than  the  periods  of  life  (in  this  or  that 
planet)  with  which  they  would  alternate. 

If  we  apply  this  conclusion  to  the  case  of  any  given  star 
or  sun  with  its  scheme  of  dependent  worlds,  we  see  that 
even  for  a  solar  system  so  selected  at  random  the  proba- 
bility of  the  existence  of  life  is  small.  It  is,  of  course, 
greater  than  for  a  single  world  taken  at  random — just  as 
if  I  had  ten  friends  who  were  to  be  at  home  each  for  six 
minutes  between  noon  and  ten,  the  chance  would  be  greater 
that  some  one  of  the  number  would  be  at  home  at  a  given 
moment  of  that  interval  than  would  be  the  chance  that  a 
given  one  of  the  number  would  be  then  at  home;  while 
yet  even  taking  all  the  ten  it  would  still  be  more  likely  than 
not  that  at  that  moment  not  one  would  be  at  home. 

Thus  when  we  look  at  any  star,  we  may  without  im- 
probability infer  that  at  the  moment  that  star  is  not  sup- 
porting life  in  any  one  of  those  worlds  which  probably 
circle  round  it. 

Have  we  then  been  led  to  the  Whewellite  theory  that 
our  earth  is  the  sole  abode  of  life?  Far  from  it.  For  not 
only  have  we  adopted  a  method  of  reasoning  which  teaches 
us  to  regard  every  planet  in  existence,  every  moon,  every 
sun,  every  orb  in  fact  in  space,  as  having  its  period  as  the 
abode  of  life,  but  the  very  argument  from  probability  which 
leads  us  to  regard  any  given  sun  as  not  the  centre  of  a 


102  PROCTOR 

scheme  in  which  at  this  moment  there  is  life,  forces  upon 
us  the  conclusion  that  among  the  millions  on  millions,  nay, 
the  millions  of  millions  of  suns  which  people  space,  mil- 
lions have  orbs  circling  round  them  which  are  at  this  pres- 
ent time  the  abode  of  living  creatures.  If  the  chance  is 
one  in  a  thousand  in  the  case  of  each  particular  star,  then 
in  the  whole  number  (practically  infinite)  of  stars,  one  in 
a  thousand  has  life  in  the  system  which  it  rules  over:  and 
what  is  this  but  saying  that  millions  of  stars  are  life-sup- 
porting orbs?  There  is  then  an  infinity  of  life  around  us, 
although  we  recognise  infinity  of  time  as  well  as  infinity  of 
space  as  an  attribute  of  the  existence  of  life  in  the  universe. 
And  remembering  that  as  life  in  each  individual  is  finite,  in 
each  planet  finite,  in  each  solar  system  finite,  and  in  each  sys- 
tem of  stars  finite,  so  (to  speak  of  no  higher  orders)  the  infin- 
ity of  life  itself  demonstrates  the  infinity  of  barrenness,  the 
infinity  of  habitable  worlds  implies  the  infinity  of  worlds  not 
as  yet  habitable,  or  which  have  long  since  passed  their 
period  of  inhabitability.  Yet  is  there  no  waste,  whether  of 
time,  of  space,  of  matter,  or  of  force;  for  waste  implies  a 
tending  toward  a  limit,  and  therefore  of  these  infinities, 
which  are  without  limits,  there  can  be  no  waste. 


MATHEMATICAL  THEORIES 
OF  THE  EARTH 

BY 

ROBERT   SIMPSON   WOODWARD 


THE  MATHEMATICAL  THEORIES  OF 
THE   EARTH1 

THE  name  of  this  section,  which  by  your  courtesy  it 
is  my  duty  to  address  to-day,  implies  a  community 
of  interest  among  astronomers  and  mathematicians. 
This  community  of  interest  is  not  difficult  to  explain.  We 
can,  of  course,  imagine  a  considerable  body  of  astronomical 
facts  quite  independent  of  mathematics.  We  can  also  im- 
agine a  much  larger  body  of  mathematical  facts  quite  inde- 
pendent of  and  isolated  from  astronomy.  But  we  never 
think  of  astronomy  in  the  large  sense  without  recognising 
its  dependence  on  mathematics,  and  we  never  think  of 
mathematics  as  a  whole  without  considering  its  capital 
applications  in  astronomy. 

Of  all  the  subjects  and  objects  of  common  interest  to 
us,  the  earth  will  easily  rank  first.  The  eart'h  furnishes  us 
with  a  stable  foundation  for  instrumental  work  and  a  fixed 
line  of  reference,  whereby  it  is  possible  to  make  out  the 
orderly  arrangement  and  procession  of  our  solar  system 
and  to  gain  some  inkling  of  other  systems  which  lie  within 
telescopic  range.  The  earth  furnishes  us  with  a  most  at- 
tractive store  of  real  problems;  its  shape,  its  size,  its  mass, 
its  precession  and  nutation,  its  internal  heat,  its  earthquakes 
and  volcanoes,  and  its  origin  and  destiny,  are  to  be  classed 
with  the  leading  questions  for  astronomical  and  mathe- 
matical research.  We  must  of  course  recognise  the  claims 
of  our  friends  the  geologists  to  that  indefinable  something 

1  Vice-presidential  address  before  the  Section  of  Mathematics  and 
Astronomy  of  the  American  Association  for  the  Advancement  of  Sci- 
ence at  the  Toronto  meeting,  August,  1889.  (From  the  "  Proceedings 
of  the  American  Association  for  the  Advancement  of  Science,"  vol. 
xxxviii.) 

105 


I06  WOODWARD 

called  the  earth's  crust,  but  considered  in  its  entirety  and 
in  its  relations  to  similar  bodies  of  the  universe,  the  earth 
has  long  been  the  special  province  of  astronomers  and 
mathematicians.  Since  the  times  of  Galileo  and  Kepler 
and  Copernicus  it  has  supplied  a  perennial  stimulus  to 
observation  and  investigation,  and  it  promises  to  tax  the 
resources  of  the  ablest  observers  and  analysts  for  some  cen- 
turies to  come.  The  mere  mention  of  the  names  of  New- 
ton, Bradley,  D'Alembert,  Laplace,  Fournier,  Gauss,  and 
Bessel,  calls  to  mind  not  only  a  long  list  of  inventions  and 
discoveries,  but  the  most  important  parts  of  mathematical 
literature.  In  its  dynamical  and  physical  aspects  the  earth 
was  to  them  the  principal  object  of  research,  and  the  thor- 
oughness and  completeness  of  their  contributions  toward 
an  explanation  of  the  "  system  of  the  world  "  are  still  a 
source  of  wonder  and  admiration  to  all  who  take  the 
trouble  to  examine  their  works. 

A  detailed  discussion  of  the  known  properties  of  the 
earth,  and  of  the  hypotheses  concerning  the  unknown 
properties,  is  no  fit  task  for  a  summer  afternoon;  the  in- 
tricacies and  delicacies  of  the  subject  are  suitable  only  for 
another  season  and  a  special  audience.  But  it  has  seemed 
that  a  somewhat  popular  review  of  the  state  of  our  mathe- 
matical knowledge  of  the  earth  might  not  be  without  in- 
terest to  those  already  familiar  with  the  complex  details, 
and  might  also  help  to  increase  that  general  interest  in  sci- 
ence, the  promotion  of  which  is  one  of  the  most  important 
functions  of  this  association. 

As  we  look  back  through  the  light  of  modern  analysis, 
it  seems  strange  that  the  successors  of  Newton,  who  took 
up  the  problem  of  the  shape  of  the  earth,  should  have 
divided  into  hostile  camps  over  the  question  whether  our 
planet  is  elongated  or  flattened  at  the  poles.  They  agreed 
in  the  opinion  that  the  earth  is  a  spheroid,  but  they  de- 
bated, investigated,  and  observed  for  nearly  half  a  century 
before  deciding  that  the  spheroid  is  oblate  rather  than 
oblong.  This  was  a  critical  question,  and  its  decision  marks 
perhaps  the  most  important  epoch  in  the  history  of  the 


MATHEMATICAL  THEORIES  OF  THE  EARTH 


107 


figure  of  the  earth.  The  Newtonian  view  of  the  oblate  form 
found  its  ablest  supporters  in  Huygens,  Maupertuis,  and 
Clairaut,  while  the  erroneous  view  was  maintained  with 
great  vigour  by  the  justly  distinguished  Cassinian  school 
of  astronomers.  Unfortunately  for  the  Cassinians,  defect- 
ive measures  of  a  meridional  arc  in  France  gave  colour 
to  the  false  theory  and  furnished  one  of  the  most  conspicu- 
ous instances  of  the  deterring  effect  of  an  incorrect  obser- 
vation. As  you  well  know,  the  point  was  definitely  settled 
by  Maupertuis's  measurement  of  the  Lapland  arc.  For 
this  achievement  his  name  has  become  famous  in  literature 
as  well  as  in  science,  for  his  friend  Voltaire  congratulated 
him  on  having  "  flattened  the  poles  and  the  Cassinis  ";  and 
Carlyle  has  honoured  him  with  the  title  of  "  Earth-flat- 
tener."  2 

Since  the  settlement  of  the  question  of  the  form — 
progress  toward  a  knowledge  of  the  size  of  the  earth  has 
been  consistent  and  steady,  until  now  it  may  be  said  that 
there  are  few  objects  with  which  we  have  to  deal  whose 
dimensions  are  so  well  known  as  the  dimensions  of  the 
earth.  But  this  is  a  popular  statement,  and,  like  most  such, 
needs  to  be  explained  in  order  not  to  be  misunderstood. 
Both  the  size  and  shape  of  the  earth  are  defined  by  the 
lengths  of  its  equatorial  and  polar  axes;  and,  knowing  the 
fact  of  the  oblate  spheroidal  form,  the  lengths  of  the  axes 
may  be  found  within  narrow  limits  from  simple  measure- 
ments conducted  on  the  surface  quite  independently  of  any 
knowledge  of  the  interior  constitution  of  the  earth.  It  is 
evident,  in  fact,  without  recourse  to  mathematical  details, 
that  the  length  of  any  arc,  as  a  degree  of  latitude  or  longi- 
tude on  the  earth's  surface,  must  depend  on  the  lengths  of 
those  axes.  Conversely,  it  is  plain  that  the  measurement 
of  such  an  arc  and  the  determination  of  its  geographical 
position  constitute  an  indirect  measurement  of  the  axes. 
Hence  it  has  happened  that  scientific  as  distinguished  from 
practical  geodesy  has  been  concerned  chiefly  with  such 

s  Todhunter,  "  History  of  the  Theories  of  Attraction  and  the  Figure 
of  the  Earth,"  London,  1873,  vol.  i,  art.  195. 


I0g  WOODWARD 

linear  and  astronomical  measurements,  and  the  zeal  with 
which  the  work  has  been  pursued  is  attested  by  triangula- 
tions  on  every  continent.  Passing  over  the  earlier  deter- 
minations as  of  historical  interest  only,  all  of  the  really 
trustworthy  approximations  to  the  lengths  of  the  axes  have 
been  made  within  the  half  century  just  passed.  The  first 
to  appear  of  these  approximations  were  the  well-founded 
values  of  Airy,3  published  in  1830.  These,  however,  were 
almost  wholly  overshadowed  and  supplanted  eleven  years 
later  by  the  values  of  Bessel,4  whose  spheroid  came  to 
occupy  a  most  conspicuous  place  in  geodesy  for  more  than 
a  quarter  of  a  century.  Knowing  as  we  now  do  that  Bes- 
sel's  values  were  considerably  in  error,  it  seems  not  a  little 
remarkable  that  they  should  have  been  so  long  accepted 
without  serious  question.  One  obvious  reason  is  found 
in  the  fact  that  a  considerable  lapse  of  time  was  essential  for 
the  accumulation  of  new  data,  but  two  other  possible  rea- 
sons of  a  different  character  are  worthy  of  notice  because 
they  are  interesting  and  instructive,  whether  specially  ap- 
plicable to  this  particular  case  or  not.  It  seems  not  im- 
probable that  the  close  agreement  of  the  values  of  Airy  and 
Bessel,  computed  independently  and  by  different  methods 
— the  greatest  discrepancy  being  about  one  hundred  and 
fifty  feet — may  have  been  incautiously  interpreted  as  a  con- 
firmation of  Bessel's  dimensions,  and  hence  led  to  their 
too  ready  adoption.  It  seems  also  not  improbable  that  the 
weight  of  Bessel's  great  name  may  have  been  too  closely 
associated  in  the  minds  of  his  followers  with  the  weights 
of  his  observations  and  results.  The  sanction  of  eminent 
authority,  especially  if  there  is  added  to  it  the  stamp  of 
an  official  seal,  is  sometimes  a  serious  obstacle  to  real 
progress.  We  can  not  do  less  than  accord  to  Bessel  the 
first  place  among  the  astronomers  and  geodesists  of  his 
day,  but  this  is  no  adequate  justification  for  the  exaggerated 
estimate  long  entertained  of  the  precision  of  the  elements 
of  his  spheroid. 

The  next  step  in  the  approximation  was  the  important 

8  "  Encyclopedia  Metropolitana." 

4  "  Astronomische  Nachrichten,"  No.  438,  1841. 


MATHEMATICAL  THEORIES  OF  THE  EARTH       109 

one  of  Clarke  5  in  1866.  His  new  values  showed  an  in- 
crease over  Bessel's  of  about  half  a  mile  in  the  equatorial 
semi-axis  and  about  three  tenths  of  a  mile  in  the  polar 
semi-axis.  Since  1866,  General  Clarke  has  kept  pace  with 
the  accumulating  data  and  given  us  so  many  different  ele- 
ments for  our  spheroid  that  it  is  necessary  to  affix  a  date 
to  any  of  his  values  we  may  use.  The  later  values,  how- 
ever, differ  but  slightly  from  the  earlier  ones,  so  that  the 
spheroid  of  1866,  which  has  come  to  be  pretty  generally 
adopted,  seems  likely  to  enjoy  a  justly  greater  celebrity 
than  that  of  its  immediate  predecessor.  The  probable  error 
of  the  axes  of  this  spheroid  is  not  much  greater  than  the 
hundred  thousandth  part,6  and  it  is  not  likely  that  new  data 
will  change  their  lengths  by  more  than  a  few  hundred  feet. 
In  the  present  state  of  science,  therefore,  it  may  be  said 
that  the  first  order  of  approximation  to  the  form  and 
dimensions  of  the  earth  has  been  successfully  attained. 
The  question  which  follows  naturally  and  immediately  is, 
How  much  further  can  the  approximation  be  carried? 
The  answer  to  this  question  is  not  yet  written,  and  the 
indications  are  not  favourable  for  its  speedy  announcement. 
The  first  approximation,  as  wre  have  seen,  requires  no  knowl- 
edge of  the  interior  density  and  arrangement  of  the  earth's 
mass;  it  proceeds  on  the  simple  assumption  that  the  sea  sur- 
face is  closely  spheroidal.  The  second  approximation,  if  it 
be  more  than  a  mere  interpolation  formula,  requires  a  knowl- 
edge of  both  density  and  arrangement  of  the  constituents  of 
the  earth's  mass,  and  especially  of  that  part  called  the  crust. 
"  All  astronomy/'  says  Laplace,  "  rests  on  the  stability  of 
the  earth's  axis  of  rotation."  7  In  a  similar  sense  we  may 
say  all  geodesy  rests  on  the  direction  of  the  plumb  line. 
The  simple  hypothesis  of  a  spheroidal  form  assumes  that 

8 "  Comparison  of  Standards  of  Length,"  made  at  the  ordnance 
office,  Southampton,  England,  by  Captain  A.  R.  Clarke,  R.  E.  Pub- 
lished by  order  of  the  Secretary  of  State  for  War,  1866. 

a  Clarke,  Colonel  A.  R.,  "  Geodesy,"  Oxford.  1880,  p.  319. 

7 "  Toute  1'Astronomie  repose  sur  rinvariabilite  de  1'axe  de  rotation 
de  la  Terre  a  la  surface  du  spheroide  terrestre  et  sur  1'uniformite  de 
cette  rotation."  ("  Mecanique  Celeste,"  Paris,  1882,  tome  v,  p.  22.) 


1 10  WOODWARD 

the  plumb  line  is  everywhere  coincident  with  the  normal  to 
the  spheroid,  or  that  the  surface  of  the  spheroid  coincides 
with  the  level  of  the  sea.  But  this  is  not  quite  correct. 
The  plumb  line  is  not  in  general  coincident  with  the  normal, 
and  the  actual  sea  level  or  geoid  must  be  imagined  to  be 
an  irregular  surface  lying  partly  above  and  partly  below  the 
ideal  spheroidal  surface.  The  deviations,  it  is  true,  are  rela- 
tively small,  but  they  are  in  general  much  greater  than  the 
unavoidable  errors  of  observation  and  they  are  the  exact 
numerical  expression  of  our  ignorance  in  this  branch  of 
geodesy.  It  is  well  known,  of  course,  that  deflections  of 
the  plumb  line  can  sometimes  be  accounted  for  by  visible 
masses,  but  on  the  whole  it  must  be  admitted  that  we  pos- 
sess only  the  vaguest  notions  of  their  cause  and  a  most 
inadequate  knowledge  of  their  distribution  and  extent. 

What  is  true  of  plumb-line  deflections  is  about  equally 
true  of  the  deviations  of  the  intensity  of  gravity  from  what 
may  be  called  the  spheroidal  type.  Given  a  closely  sphe- 
roidal form  of  the  sea  level  and  it  follows  from  the  law  of 
gravitation,  as  a  first  approximation,  without  any  knowl- 
edge of  the  distribution  of  the  earth's  mass,  that  the  in- 
crease of  gravity  varies  as  the  square  of  the  sine  of  the 
latitude  in  passing  from  the  equator  to  the  poles.  This 
is  the  remarkable  theorem  of  Stokes,8  and  it  enables  us  to 
determine  the  form  or  ellipticity  of  the  earth  by  means  of 
pendulum  observations  alone.  It  must  be  admitted,  how- 
ever, that  the  values  of  the  ellipticity  recently  obtained  in 
this  way  by  the  highest  authorities,  Clarke  9  and  Helmert,10 
are  far  from  satisfactory,  whether  we  regard  them  in  the 
light  of  their  discrepancy  or  in  the  light  of  the  different 
methods  of  computing  them.  In  general  terms  we  may 
say  that  the  difficulty  in  the  way  of  the  use  of  pendulum 
observations  still  hinges  on  the  treatment  of  local  anomalies 
and  on  the  question  of  reduction  to  sea  level.  At  present, 

8  Stokes,  G.  G.,  "  Mathematical  and  Physical  Papers,"  Cambridge 
University  Press,  1880,  vol.  ii. 

'"Geodesy,"  chap.  xiv. 

10  Helmert,  Dr.  F.  R.,  "  Die  Mathematischen  und  Physikalischen 
Theorieen  der  Hoheren  Geodasie,"  Leipsic,  1880,  1884,  ii  Teil. 


MATHEMATICAL  THEORIES  OF  THE  EARTH   m 

the  case  is  one  concerning  which  the  doctors  agree  neither 
in  their  diagnosis  nor  in  their  remedies. 

Turning  attention  now  from  the  surface  toward  the  in- 
terior, what  can  be  said  of  the  earth's  mass  as  a  whole,  of 
its  laws  of  distribution,  and  of  the  pressures  that  exist  at 
great  depths?  Two  facts — namely,  the  mean  density  and 
the  surface  density — are  roughly  known;  a  third  fact — 
namely,  the  precession  constant,  or  the  ratio  of  the  differ- 
ence of  the  two  principal  moments  of  inertia  to  the  greater 
of  them — is  known  with  something  like  precision.  These 
facts  lie  within  the  domain  of  observation  and  require  only 
the  law  of  gravitation  for  their  verification.  Certain  in- 
ferences, also,  from  these  facts  and  others,  have  long  been 
and  still  are  held  to  be  hardly  less  cogent  and  trustworthy, 
but  before  stating  them  it  will  be  well  to  recall  briefly  the 
progress  of  opinion  concerning  this  general  subject  during 
the  past  century  and  a  half. 

The  conception  of  the  earth  as  having  been  primitively 
fluid  was  the  prevailing  one  among  mathematicians  before 
Clairaut  published  his  "  Theorie  de  la  Figure  de  la  Terre," 
in  1743.  By  the  aid  of  this  conception  Clairaut  proved 
the  celebrated  theorem  which  bears  his  name,  and  probably 
no  idea  in  the  mechanics  of  the  earth  has  been  more  sug- 
gestive and  fruitful.  It  was  the  central  idea  in  the  elaborate 
investigations  of  Laplace  and  received  at  his  hands  a  de- 
velopment which  his  successors  have  found  it  about  equally 
difficult  to  displace  or  to  improve.  From  the  idea  of 
fluidity  spring  naturally  the  hydrostatical  notions  of  pres- 
sure and  level  surfaces,  or  the  arrangement  of  fluid  masses 
in  strata  of  uniform  density.  Hence  follows,  also,  the  no- 
tion of  continuity  of  increase  in  density  from  the  surface 
toward  the  centre  of  the  earth.  All  of  the  principal  me- 
chanical properties  and  effects  of  the  earth's  mass — viz., 
the  ellipticity,  the  surface  density,  the  mean  density,  the 
precession  constant,  and  the  lunar  inequalities — were  cor- 
related by  Laplace  11  in  a  single  hypothesis,  involving  only 
one  assumption  in  addition  to  that  of  original  fluidity  and 
11 "  Mecanique  Celeste,"  tome  v,  livre  xi. 


112  WOODWARD 

the  law  of  gravitation.  This  assumption  relates  to  the  com- 
pressibility of  matter  and  asserts  that  the  ratio  of  the  in- 
crement of  pressure  to  the  increment  of  density  is  propor- 
tional to  the  density.  Many  interesting  and  striking  con- 
clusions follow  readily  from  this  hypothesis,  but  the  most 
interesting  and  important  are  those  relative  to  density  and 
pressure,  especially  the  latter,  whose  dominance  as  a  factor 
in  the  mechanics  of  celestial  masses  seems  destined  to 
survive  whether  the  hypothesis  stands  or  falls.  The  hy- 
pothesis requires  that,  while  the  density  increases  slowly 
from  something  less  than  three  at  the  surface  to  about 
eleven  at  the  centre  of  the  earth,  the  pressure  within  the 
mass  increases  rapidly  below  the  surface,  reaching  a  value 
surpassing  the  crushing  strength  of  steel  at  the  depth  of 
a  few  miles  and  amounting  at  the  centre  to  no  less  than 
three  million  atmospheres.  The  inferences,  then,  as  dis- 
tinguished from  facts,  are  that  the  mass  of  the  earth  is  very 
nearly  symmetrically  disposed  about  its  centre  of  gravity, 
that  pressure  and  density  except  near  the  surface  are 
mutually  dependent,  and  that  the  earth  in  reaching  this 
stage  has  passed  through  the  fluid  or  quasi-fluid  state. 

Later  writers  have  suggested  other  hypotheses  for  a 
continuous  distribution  of  the  earth's  mass,  but  none  of 
them  can  be  said  to  rival  the  hypothesis  of  Laplace.  Their 
defects  lie  either  in  not  postulating  a  direct  connection  be- 
tween density  and  pressure  or  in  postulating  a  connection 
which  implies  extreme  or  impossible  values  for  these  and 
other  mechanical  properties  of  the  mass. 

It  is  clear,  from  the  positiveness  of  his  language  in 
frequent  allusions  to  this  conception  of  the  earth,  that  La- 
place was  deeply  impressed  with  its  essential  correctness. 
"  Observations/'  he  says,  "  prove  incontestably  that  the 
densities  of  the  strata  (couches)  of  the  terrestrial  spheroid 
increase  from  the  surface  to  the  centre,"  12  and  "  the  regu- 

u  "  Enfin  il  (Newton)  regarde  la  terre  comme  homogene,  ce  qui  est 
contraire  aux  observations,  qui  prouvent.  incontestablement  que  les 
densites  des  couches  du  spheroide  terrestre  croissent  de  la  surface 'au 
centre."  ("  Mecanique  Celeste,"  tome  v,  p.  9.) 


MATHEMATICAL  THEORIES  OF  THE  EARTH 

larity  with  which  the  observed  variation  in  length  of  a 
second's  pendulum  follows  the  law  of  squares  of  the  sines 
of  the  latitudes  proves  that  the  strata  are  arranged  sym- 
metrically about  the  centre  of  gravity  of  the  earth."  13 
The  more  recent  investigations  of  Stokes,  to  which  allu- 
sion has  already  been  made,  forbid  our  entertaining  any- 
thing like  so  confident  an  opinion  of  the  earth's  primi- 
tive fluidity  or  of  a  symmetrical  and  continuous  arrange- 
ment of  its  strata.  But,  though  it  must  be  said  that 
the  sufficiency  of  Laplace's  arguments  has  been  seriously 
impugned,  we  can  hardly  think  the  probability  of  the 
correctness  of  his  conclusions  has  been  proportionately 
diminished. 

Suppose,  however,  that  we  reject  the  idea  of  original 
fluidity.  Would  not  a  rotating  mass  of  the  size  of  the  earth 
assume  finally  the  same  aspects  and  properties  presented  by 
our  planet?  Would  not  pressure  and  centrifugal  force  suf- 
fice to  bring  about  a  central  condensation  and  a  sym- 
metrical arrangement  of  strata  similar  at  least  to  that  re- 
quired by  the  Laplacian  hypothesis?  Categorical  answers 
to  these  questions  can  not  be  given  at  present.  But,  what- 
ever may  have  been  the  antecedent  condition  of  the  earth's 
mass,  the  conclusion  seems  unavoidable  that  at  no  great 
depth  the  pressure  is  sufficient  to  break  down  the  struc- 
tural characteristics  of  all  known  substances,  and  hence 
to  produce  viscous  flow  whenever  and  wherever  the  stress 
difference  exceeds  a  certain  limit,  which  can  not  be  large 
in  comparison  with  the  pressure.  Purely  observational  evi- 
dence, also,  of  a  highly  affirmative  kind  in  support  of  this 
conclusion,  is  afforded  6y  the  remarkable  results  of  Tresca's 
experiments  on  the  flow  of  solids  and  by  the  abundant 
proofs  in  geology  of  the  plastic  movements  and  viscous 
flow  of  rocks.  With  such  views  and  facts  in  mind  the  fluid 
stage,  considered  indispensable  by  Laplace,  does  not  appear 

"  "  La  regularite  avec  laquelle  la  variation  observee  des  longueurs 
du  pendule  a  secondes  suit  la  loi  du  carre  du  sinus  de  la  latitude  prouve 
que  ces  couches  sont  disposees  regulierement  autour  du  centre  de 
gravite  de  la  terre  et  que  leur  forme  est  a  peu  pres  elliptique  et  de  revo- 
lution." ("  Mecanique  Celeste,"  tome  v,  p.  17.) 
8 


H4  WOODWARD 

necessary  to  the  evolution  of  a  planet,  even  if  it  reach  the 
extreme  refinement  of  a  close  fulfilment  of  some  such 
mathematical  law  as  that  of  his  hypothesis.  If,  as  is  here 
assumed,  pressure  be  the  dominant  factor  in  such  large 
masses,  the  attainment  of  a  stable  distribution  would  be 
simply  a  question  of  time.  The  fluid  mass  might  take  on 
its  normal  form  in  a  few  days  or  a  few  months,  whereas  the 
viscous  mass  might  require  a  few  thousand  or  a  few  mil- 
lion years. 

Some  physicists  and  mathematicians,  on  the  other  hand, 
reject  both  the  idea  of  existence  of  great  pressures  within 
the  earth's  mass,  and  the  notion  of  an  approach  to  con- 
tinuity in  the  distribution  of  density.  As  representing  this 
side  of  the  question  the  views  of  the  late  M.  Roche,  who 
wrote  much  on  the  constitution  of  the  earth,  are  worthy  of 
consideration.  He  tells  us  that  the  very  magnitude  of  the 
central  pressure  computed  on  the  hypothesis  of  fluidity  is 
itself  a  peremptory  objection  to  that  hypothesis.14  Accord- 
ing to  his  conception,  the  strata  of  the  earth  from  the 
centre  outward  are  substantially  self-supporting  and  un- 
yielding. It  does  not  appear,  however,  that  he  had  sub- 
mitted this  conception  to  the  test  of  numbers,  for  a  simple 
calculation  will  show  that  no  materials  of  which  we  have 
any  knowledge  would  sustain  the  stress  in  such  shells  or 
domes.  If  the  crust  of  the  earth  were  self-supporting,  its 
crushing  strength  would  Have  to  be  about  thirty  times  that 
of  the  best  cast  steel,  or  five  hundred  to  one  thousand  times 
that  of  granite.  The  views  of  Roche  on  the  distribution  of 
the  terrestrial  densities  appear  equally  extreme.15  He  pre- 
fers to  consider  the  mass  as  made  up  of  two  distinct  parts, 
an  outer  shell  or  crust  whose  thickness  is  about  one  sixth 
of  the  earth's  radius,  and  a  solid  nucleus  having  little  or  no 
central  condensation.  The  nucleus  is  conceived  to  be 
purely  metallic,  and  to  have  about  the  same  density  as 
iron.  To  account  for  geological  phenomena,  he  postulates 

14 "  Memoire  stir  1'etat  interieur  du  globe  terrestre,"  par  M.  £douard 
Roche;  "  Memoires  de  la  section  des  sciences  de  1' Academic  des  Sci- 
ences et  Lettres  de  Montpellier,"  1880-1884,  tome  x.  "  Ibid. 


MATHEMATICAL  THEORIES   OF  THE   EARTH       115 

a  zone  of  fusion  separating  the  crust  from  the  nucleus. 
The  whole  hypothesis  is  consistently  worked  out  in  con- 
formity with  the  requirements  of  the  ellipticity,  the  super- 
ficial density,  the  mean  density,  and  precession;  so  that  to 
one  who  can  divest  his  mind  of  the  notion  that  pressure 
and  continuity  are  important  factors  in  the  mechanics  of 
such  masses,  the  picture  which  Roche  draws  out  of  the 
constitution  of  our  planet  will  present  nothing  incon- 
gruous. 

In  a  field  so  little  explored  and  so  inaccessible,  though 
hedged  about  as  we  have  seen  by  certain  sharply  limiting 
conditions,  there  is  room  for  a  wide  range  of  opinion  and 
for  great  freedom  in  the  play  of  hypothesis;  and  although 
the  preponderance  of  evidence  appears  to  be  in  favour  of 
a  terrestrial  mass  in  which  the  reign  of  pressure  is  well- 
nigh  absolute,  we  should  not  be  surprised  a  few  decades 
or  centuries  hence  to  find  many  of  our  notions  on  this  sub- 
ject radically  defective. 

If  the  problem  of  the  constitution  and  distribution  of 
the  earth's  mass  is  yet  an  obscure  and  difficult  one  after 
two  centuries  of  observation  and  investigation,  can  we  re- 
port any  greater  degree  of  success  in  the  treatment  of  that 
still  older  problem  of  the  earth's  internal  heat;  of  its  origin 
and  effects?  Concerning  phenomena  always  so  impressive 
and  often  so  terribly  destructive  as  those  intimately  con- 
nected with  the  terrestrial  store  of  heat,  it  is  natural  that 
there  should  be  a  considerable  variety  of  opinion.  The 
consensus  of  such  opinion,  however,  has  long  been  in  fa- 
vour of  the  hypothesis  that  heat  is  the  active  cause  of  many 
and  a  potent  factor  in  most  of  the  grander  phenomena 
which  geologists  assign  to  the  earth's  crust;  and  the  pre- 
vailing interpretation  of  these  phenomena  is  based  on  the 
assumption  that  our  planet  is  a  cooling  sphere  whose  outer 
shell  or  crust  is  constantly  cracked  and  crumpled  in  adjust- 
ing itself  to  the  shrinking  nucleus. 

The  conception  that  the  earth  was  originally  an  in- 
tensely heated  and  molten  mass  appears  to  have  first  taken 
something  like  definite  form  in  the  minds  of  Leibnitz  and 


Il6  WOODWARD 

Descartes.16  But  neither  of  these  philosophers  was  armed 
with  the  necessary  mathematical  equipment  to  subject  this 
conception  to  the  test  of  numerical  calculation.  Indeed, 
it  was  not  fashionable  in  their  day,  any  more  than  it  is  with 
some  philosophers  in  ours,  to  undertake  the  drudgery  of 
applying  the  machinery  of  analysis  to  the  details  of  a 
hypothesis.  Nearly  a  century  elapsed  before  an  order  of 
intellects  capable  of  dealing  with  this  class  of  questions  ap- 
peared. It  was  reserved  for  Joseph  Fournier  to  lay  the 
foundation  and  build  a  great  part  of  the  superstructure  of 
our  modern  theory  of  heat  diffusion,  his  avowed  desire 
being  to  solve  the  great  problem  of  terrestrial  heat.  "  The 
question  of  terrestrial  temperatures,"  he  says,  "  has  always 
appeared  to  us  one  of  the  grandest  objects  of  cosmological 
studies,  and  we  have  had  it  principally  in  view  in  estab- 
lishing the  mathematical  theory  of  heat."  17  This  ambition, 
however,  was  only  partly  realized.  Probably  Fournier 
underestimated  the  difficulties  of  his  problem,  for  his  most 
ingenious  and  industrious  successors  in  the  same  field  have 
made  little  progress  beyond  the  limits  he  attained.  But 
the  work  he  left  is  a  perennial  index  to  his  genius.  Though 
quite  inadequately  appreciated  by  his  contemporaries,  the 
"  Analytical  Theory  of  Heat,"  which  appeared  in  1820,  is 
now  conceded  to  be  one  of  the  epoch-making  books.  In- 
deed, to  one  who  has  caught  the  spirit  of  the  extraordinary 
analysis  which  Fournier  developed  and  illustrated  by  nu- 
merous applications  in  this  treatise,  it  is  evident  that  he 
opened  a  field  whose  resources  are  still  far  from  being  ex- 
hausted. A  little  later  Poisson  took  up  the  same  class  of 
questions  and  published  another  great  work  on  the  mathe- 
matical theory  of  heat.18  Poisson  narrowly  missed  being 

18  Protegee,  ou  de  la  formation  et  des  revolutions  du  globe,  par 
Leibnitz,  ouvrage  traduite  .  .  .  avec  tine  introduction  et  des  notes 
par  le  Dr.  Bertrand  de  Saint-Germain,  Paris,  1859. 

17 "  La  question  des  temperatures  terrestres  nous  a  toujours  paru 
un  des  plus  grands  objets  des  etudes  cosmologiques,  et  nous  1'avions 
principalement  en  vue  en  etablissant  la  theorie  mathematique  de  la 
chaleur."  ("  Annales  de  Chimie  et  de  Physique,"  1824,  tome  xxvii, 
p.  159.^ 

18 "  Theorie  Mathematique  de  la  Chaleur,"  Paris,  1835. 


MATHEMATICAL  THEORIES  OF  THE  EARTH 

the  foremost  mathematician  of  his  day.  In  originality,  in 
wealth  of  mathematical  resources,  and  in  breadth  of  grasp 
of  physical  principles  he  was  the  peer  of  the  able'st  of  his 
contemporaries.  In  lucidity  of  exposition  it  would  be 
enough  to  say  that  he  was  a  Frenchman,  but  he  seems  to 
have  excelled  in  this  peculiarly  national  trait.  His  contri- 
butions to  the  theory  of  heat  have  been  somewhat  over- 
shadowed in  recent  times  by  the  earlier  and  perhaps  more 
brilliant  researches  of  Fournier,  but  no  student  can  afford 
to  take  up  that  enticing  though  difficult  theory  without  the 
aid  of  Poisson  as  well  as  Fournier. 

It  is  natural,  therefore,  that  we  should  inquire  what 
opinions  these  great  masters  in  the  mathematics  of  heat 
diffusion  held  concerning  the  earth's  store  of  heat.  I  say 
opinions,  for  unhappily  this  whole  subject  is  still  so  largely 
a  matter  of  opinion  that,  in  discussing  it,  one  may  not  inap- 
propriately adopt  the  famous  caution  of  Marcus  Aurelius, 
"  Remember  that  all  is  opinion."  It  does  not  appear  that 
Fournier  reached  any  definite  conclusion  on  this  question, 
though  he  seems  to  have  favoured  the  view  that  the  earth 
in  cooling  from  an  earlier  state  of  incandescence  reached 
finally  through  convection  a  condition  in  which  there  was 
a  uniform  distribution  of  heat  throughout  its  mass.  This 
is  the  consistentior  status  of  Leibnitz,  and  it  begins  with 
the  formation  of  the  earth's  crust,  if  not  with  the  consolida- 
tion of  the  entire  mass.  It  thus  affords  an  initial  distribu- 
tion of  heat  and  an  epoch  from  which  analysis  may  start, 
and  the  problem  for  the  mathematician  is  to  assign  the  sub- 
sequent distribution  of  heat  and  the  resulting  mechanical 
effects.  But  no  great  amount  of  reflection  is  necessary  to 
convince  one  that  the  analysis  can  not  proceed  without 
making  a  few  more  assumptions.  The  assumptions  which 
involve  the  least  difficulty,  and  which  for  this  reason  partly 
have  met  with  most  favour,  are  that  the  conductivity  and 
thermal  capacity  of  the  entire  mass  remain  constant,  and 
that  the  heat  conducted  to  the  surface  of  the  earth  passes 
off  by  the  combined  process  of  radiation,  convection,  and 
conduction,  without  producing  any  sensible  effect  on  sur- 


Il8  WOODWARD 

rounding  space.  These  or  similar  assumptions  must  be 
made  before  the  application  of  theory  can  begin.  In  addi- 
tion, two  data  are  essential  to  numerical  calculations — 
namely,  the  diffusivity,  or  ratio  of  the  conductivity  of  the 
mass  to  its  thermal  capacity,  and  the  initial  uniform  tem- 
perature. The  first  of  these  can  be  observed,  approxi- 
mately, at  least;  the  second  can  only  be  estimated  at  pres- 
ent. With  respect  to  these  important  points  which  must 
be  considered  after  the  adoption  of  the  consistentior  status, 
the  writings  of  Fournier  afford  little  light.  He  was  content, 
perhaps,  to  invent  and  develop  the  exquisite  analysis  requi- 
site to  the  treatment  of  such  problems. 

Poisson  wrote  much  on  the  whole  subject  of  terrestrial 
temperatures,  and  carefully  considered  most  of  the  trouble- 
some details  which  lay  between  his  theory  and  its  applica- 
tion. While  he  admitted  the  nebular  hypothesis  and  an 
initial  fluid  state  of  the  earth,  he  rejected  the  notion  that 
the  observed  increase  of  underground  temperature  is  due 
to  a  primitive  store  of  heat.  If  the  earth  was  originally 
fluid  by  reason  of  its  heat,  a  supposition  which  Poisson  re- 
garded quite  gratuitous,  he  conceived  that  it  must  cool  and 
consolidate  from  the  centre  outward;19  so  that  according 
to  this  view  the  crust  of  our  planet  arrived  at  a  condition 
of  stability  only  after  the  supply  of  heat  had  been  exhausted. 
But  Poisson  was  not  at  a  loss  to  account  for  the  observed 
temperature  gradient  in  the  earth's  crust.  Always  fertile  in 
hypotheses,  he  advanced  the  idea  that  there  exists,  by  rea- 
son of  interstellar  radiations,  great  variations  in  the  tem- 
perature of  space,  some  vast  regions  being  comparatively 
cool  and  others  intensely  hot,  and  that  the  present  store  of 
terrestrial  heat  was  acquired  by  a  journey  of  the  solar  sys- 
tem through  one  of  the  hotter  regions.  "  Such  is,"  he  says, 
"  in  my  opinion,  the  true  cause  of  the  augmentation  of 
temperature  which  occurs  as  we  descend  below  the  surface 
of  the  globe."  20  This  hypothesis  was  the  result  of  Pois- 

19 "  Theorie  Mathematique  de  la  Chaleur,"  Supplement  de,  Paris, 
1837. 

20 "  Telle  est,  dans  mon  opinion,  la  cause  veritable  de  1'augmentation 


MATHEMATICAL  THEORIES  OF  THE  EARTH 

son's  mature  reflection,  and  as  such  is  well  worthy  of  atten- 
tion. The  notion  that  there  exist  hot  foci  in  space  was 
advanced  also  in  another  form  in  1852  by  Rankine,  in  his 
interesting  speculation  on  the  reconcentration  of  energy. 
But  whatever  we  may  think  of  the  hypothesis  as  a  whole, 
it  does  not  appear  to  be  adequate  to  the  case  of  the  earth 
unless  we  suppose  the  epoch  of  transit  through  the  hot 
region  exceedingly  remote  and  the  temperature  of  that 
region  exceedingly  high.  The  continuity  of  geological  and 
paleontological  phenomena  is  much  better  satisfied  by  the 
Leibnitzian  view  of  an  earth  long  subject  to  comparatively 
constant  surface  conditions,  but  still  active  with  the  energy 
of  its  primitive  heat. 

Notwithstanding  the  indefatigable  and  admirable  la- 
bours of  Fournier  and  Poisson  in  this  field,  it  must  be 
admitted  that  they  accomplished  little  more  than  the  prepa- 
ration of  the  machinery  with  which  their  successors  have 
sought  and  are  still  seeking  to  reap  the  harvest.  The  dif- 
ficulties which  lay  in  their  way  were  not  mathematical  but 
physical.  Had  they  been  able  to  make  out  the  true  condi- 
tions of  the  earth's  store  of  heat,  they  would  undoubtedly 
have  reached  a  high  grade  of  perfection  in  the  treatment  of 
the  problem.  The  theory  as  they  left  it  was  much  in 
advance  of  observation,  and  the  labours  of  their  successors 
have  therefore  necessarily  been  directed  largely  toward  the 
determination  of  the  thermal  properties  of  the  earth's  crust 
and  mass. 

Of  those  who  in  the  present  generation  have  con- 
tributed to  our  knowledge  and  stimulated  the  investigation 
of  this  subject,  it  is  hardly  necessary  to  say  that  we  owe 
most  to  Sir  William  Thomson.  He  has  made  the  question 
of  terrestrial  temperatures  highly  attractive  and  instructive 
to  astronomers  and  mathematicians,  and  not  less  warmly  in- 
teresting to  geologists  and  paleontologists.  Whether  we 
are  prepared  to  accept  his  conclusions  or  not,  we  must  all 

de  temperature  qui  a  lieu  sur  chaque  verticale  a  mesure  que  Ton 
s'abaisse  au-dessous  de  la  surface  du  globe."  ("  Theorie  Mathematique 
de  la  Chaleur,"  Supplement  de,  p.  15.) 


I20  WOODWARD 

acknowledge  our  indebtedness  to  the  contributions  of  his 
master  hand  in  this  field,  as  well  as  in  most  other  fields  of 
terrestrial  physics.  The  contribution  of  special  interest  to 
us  in  this  connection  is  his  remarkable  memoir  on  the 
secular  cooling  of  the  earth.21  In  this  memoir  he  adopts 
the  simple  hypothesis  of  a  solid  sphere  whose  thermal 
properties  remain  invariable  while  it  cools  by  conduction 
from  an  initial  state  of  uniform  temperature,  and  draws 
therefrom  certain  striking  limitations  on  geologic  time. 
Many  geologists  were  startled  by  these  limitations,  and 
geologic  thought  and  opinion  have  since  been  widely  in- 
fluenced by  them.  It  will  be  of  interest,  therefore,  to  state 
a  little  more  fully  and  clearly  the  grounds  from  which  his 
arguments  proceed.  Conceive  a  sphere  having  a  uniform 
temperature  initially,  to  cool  in  a  medium  which  instantly 
dissipates  all  heat  brought  by  conduction  to  its  surface, 
thus  keeping  the  surface  at  a  constant  temperature.  Sup- 
pose we  have  given  the  initial  excess  of  the  sphere's  tem- 
perature over  that  of  the  medium.  Suppose  also  that  the 
capacity  of  the  mass  of  the  sphere  for  the  diffusion  of  heat 
is  known,  and  known  to  remain  invariable  during  the 
process  of  cooling.  This  capacity  is  called  diffusivity,  and 
is  a  constant  which  can  be  observed.  Then  from  these  data 
the  distribution  of  temperature  at  any  future  time  can  be 
assigned,  and  hence  also  the  rate  of  temperature  increase, 
or  the  temperature  gradient,  from  the  surface  toward  the 
centre  of  the  sphere  can  be  computed.  It  is  tolerably  cer- 
tain that  the  heat  conducted  from  the  interior  to  the  surface 
of  the  earth  does  not  set  up  any  reaction  which  in  any 
sensible  degree  retards  the  process  of  cooling.  It  escapes 
so  freely  that,  for  practical  purposes,  we  may  say  it  is  in- 
stantly dissipated.  Hence,  if  we  can  assume  that  the  earth 
had  a  specified  uniform  temperature  at  the  initial  epoch, 
and  can  assume  its  diffusivity  to  remain  constant,  the  whole 
history  of  cooling  is  known  so  soon  as  we  determine  the 
diffusivity  and  the  temperature  gradient  at  any  point. 

81 "  Transactions  of  the  Royal  Society  of  Edinburgh,"  1862.    Thom- 
son and  Tait's  "  Natural  Philosophy,"  vol.  i,  part  ii,  Appendix  D. 


MATHEMATICAL  THEORIES  OF  THE   EARTH       121 

Now,  Sir  William  Thomson  determined  a  value  for  the 
diffusivity  from  measurements  of  the  seasonal  variations 
of  underground  temperatures,  and  numerous  observations 
of  the  increase  of  temperature  with  depth  below  the  earth's 
surface  gave  an  average  value  for  the  temperature  gradient. 
From  these  elements,  and  from  an  assumed  initial  tem- 
perature of  7,000°  Fahr.,  he  infers  that  geologic  time  is 
limited  to  something  between  20,000,000  and  400,000,000 
years.  He  says:  "  We  must  allow  very  wide  limits  in  such 
an  estimate  as  I  have  attempted  to  make;  but  I  think  we 
may  with  much  probability  say  that  the  consolidation  can 
not  have  taken  place  less  than  20,000,000  years  ago,  or 
we  should  have  more  underground  heat  than  we  actually 
have,  nor  more  than  400,000,000  years  ago,  or  we  should 
not  have  so  much  as  the  least  observed  underground  incre- 
ment of  temperature.  That  is  to  say,  I  conclude  that 
Leibnitz's  epoch  of  emergence  of  the  consistentior  status 
was  probably  between  those  dates."  These  conclusions 
were  announced  twenty-seven  years  ago  and  were  repub- 
lished  without  modification  in  1883.  Recently,  also,  Pro- 
fessor Tait,  reasoning  from  the  same  basis,  has  insisted 
with  equal  confidence  on  cutting  down  the  upper  limit  of 
geologic  time  to  some  such  figures  as  10,000,000  or 
15,000,000  years.22  As  mathematicians  and  astronomers, 
we  must  all  confess  to  a  deep  interest  in  these  conclusions 
and  the  hypothesis  from  which  they  flow.  They  are  very 
important  if  true.  But  what  are  the  probabilities?  Hav- 
ing been  at  some  pains  to  look  into  this  matter,  I  feel 
bound  to  state  that,  although  the  hypothesis  appears  to  be 
the  best  which  can  be  formulated  at  present,  the  odds  are 
against  its  correctness.  Its  weak  links  are  the  unverified 
assumptions  of  an  initial  uniform  temperature  and  a  con- 
stant diffusivity.  Very  likely  these  are  approximations,  but 
of  what  order  we  can  not  decide.  Furthermore,  if  we  ac- 
cept the  hypothesis,  the  odds  appear  to  be  against  the  pres- 
ent attainment  of  trustworthy  numerical  results,  since  the 
data  for  calculation,  obtained  mostly  from  observations 
K  "  Recent  Advances  in  Physical  Science,"  London,  1876. 


122  WOODWARD 

on  continental  areas,  are  far  too  meagre  to  give  satisfac- 
tory average  values  for  the  entire  mass  of  the  earth.  In 
short,  this  phase  of  the  case  seems  to  stand  about  where 
it  did  twenty  years  ago,  when  Huxley  warned  us  that  the 
perfection  of  our  mathematical  mill  is  no  guarantee  of 
the  quality  of  the  grist,  adding  that,  "  as  the  grandest 
mill  will  not  extract  wheat  flour  from  peascods,  so  pages 
of  formulae  will  not  get  a  definite  result  out  of  loose 
data."  23 

When  we  pass  from  the  restricted  domain  of  quantita- 
tive results  concerning  geologic  time  to  the  freer  domain 
of  qualitative  results  of  a  general  character,  the  contrac- 
tional  theory  of  the  earth  may  be  said  still  to  lead  all  others, 
though  it  seems  destined  to  require  more  or  less  modifica- 
tion if  not  to  be  relegated  to  a  place  of  secondary  impor- 
tance. Old,  however,  as  is  the  notion  that  the  great  sur- 
face irregularities  of  the  earth  are  but  the  outward  evidence 
of  a  crumpling  crust,  it  is  only  recently  that  this  notion  has 
been  subjected  to  mathematical  analysis  on  anything  like 
a  rational  basis.  About  three  years  ago  Mr.  T.  Mellard 
Reade 24  announced  the  doctrine  that  the  earth's  crust 
from  the  joint  effect  of  its  heat  and  gravitation  should  be- 
have in  a  way  somewhat  analogous  to  a  bent  beam,  and 
should  possess  at  a  certain  depth  a  "  level  of  no  strain  " 
corresponding  to  the  neutral  surface  in  a  beam.  Above 
the  level  of  no  strain,  according  to  this  doctrine,  the  strata 
*will  be  subjected  to  compression  and  will  undergo  crum- 
pling, while  below  that  level  the  tendency  of  the  strata  to 
crack  and  part  is  overcome  by  pressure  which  produces 
what  Reade  calls  "  compressive  extension,"  thus  keeping 
the  nucleus  compact  and  continuous.  A  little  later  the 
same  idea  was  worked  out  independently  by  Mr.  Charles 
Davison,25  and  it  has  since  received  elaborate  mathematical 

n  "  Geological  Reform  "  ("  The  Anniversary  Address  to  the  Geo- 
logical Society  for  1869  "). 

24  Reade,  T.  Mellard,  "  Origin  of  Mountain  Ranges,"  London,  1886. 

25 "  On  the  Distribution  of  Strain  in  the  Earth's  Crust  resulting 
from  Secular  Cooling,  with  Special  Reference  to  the  Growth  of  Con- 
tinents and  the  Formation  of  Mountain  Chains."  By  Charles  Davison, 


MATHEMATICAL  THEORIES  OF  THE  EARTH      123 

treatment  at  the  hands  of  Darwin,28  Fisher,27  and  others. 
The  doctrine  requires  for  its  application  a  competent  theory 
of  cooling,  and  hence  can  not  be  depended  on  at  present 
to  give  anything  better  than  a  general  idea  of  the  me- 
chanics of  crumpling  and  a  rough  estimate  of  the  magni- 
tudes of  the  resulting  effects.  Using  Thomson's  hypothe- 
sis, it  appears  that  the  stratum  of  no  strain  moves  down- 
ward from  the  surface  of  the  earth  at  a  nearly  constant  rate 
during  the  earlier  stages  of  cooling,  but  more  slowly  dur- 
ing later  stages;  its  depth  is  independent  of  the  initial  tem- 
perature of  the  earth;  and  if  we  adopt  Thomson's  value  of 
the  diffusivity,  it  will  be  about  two  and  a  third  miles  below 
the  surface  in  100,000,000  years  from  the  beginning  of 
cooling,  and  a  little  more  than  fourteen  miles  below  the 
surface  in  700,000,000  years.  The  most  important  infer- 
ence from  this  theory  is  that  the  geological  effects  of  secu- 
lar cooling  will  be  confined  for  a  very  long  time  to  a  com- 
paratively thin  crust.  Thus  if  the  earth  is  100,000,000 
years  old,  crumpling  should  not  extend  much  deeper  than 
two  miles.  A  test  to  which  the  theory  has  been  subjected, 
and  one  which  some  2S  consider  crucial  against  it,  is  the 
volumetric  amount  of  crumpling  shown  by  the  earth  at 
the  present  time.  This  is  a  difficult  quantity  to  estimate, 
but  it  appears  to  be  much  greater  than  the  theory  can 
account  for. 

The  opponents  of  the  contractional  theory  of  the  earth, 
believing  it  quantitatively  insufficient,  have  recently  re- 
vived and  elaborated  an  idea  first  suggested  by  Babbage  29 
and  Herschel  in  explanation  of  the  greater  folds  and  move- 
ments of  the  crust.  This  idea  figures  the  crust  as  being  in 
a  state  bordering  on  hydrostatic  equilibrium,  which  can 

with  a  note  by  G.  H.  Darwin.  "  Philosophical  Transactions,"  vol. 
clxxviii  (1887),  A,  pp.  231-249. 

19  Ibid. 

"  Fisher,  Rev.  Osmond,  "  Physics  of  the  Earth's  Crust,"  second 
edition,  London,  1889,  chap.  viii. 

M  Notably,  Rev.  Osmond  Fisher.  See  his  "  Physics  of  the  Earth's 
Crust,"  chap.  viii. 

w  Appendix  to  the  "  Ninth  Bridgewater  Treatise  "  (by  C.  Babbage), 
second  edition,  London,  1838. 


124  WOODWARD 

not  be  greatly  disturbed  without  a  readjustment  and  con- 
sequent movement  of  the  masses  involved.  According  to 
this  view,  the  transfer  of  any  considerable  load  from  one 
area  to  another  is  followed  sooner  or  later  by  a  depression 
over  the  loaded  area  and  a  corresponding  elevation  over 
the  unloaded  one,  and  in  a  general  way  it  is  inferred  that 
the  elevation  of  continental  areas  tends  to  keep  pace  with 
erosion.  The  process  by  which  this  balance  is  maintained 
has  been  called  isostasy,30  and  the  crust  is  said  to  be  in  an 
isostatic  state.  The  dynamics  of  the  superficial  strata  with 
the  attendant  phenomena  of  folding  and  faulting  are  thus 
referred  to  gravitation  alone,  or  to  gravitation  and  what- 
ever opposing  force  the  rigidity  of  the  strata  may  offer. 
In  a  mathematical  sense,  however,  the  theory  of  isostasy 
is  in  a  less  satisfactory  state  than  the  theory  of  con- 
traction. As  yet  we  can  see  only  that  isostasy  is  an 
efficient  cause  if  once  set  in  action,  but  how  it  is  started 
and  to  what  extent  it  is  adequate  remain  to  be  deter- 
mined. Moreover,  isostasy  does  not  seem  to  meet  the 
requirements  of  geological  continuity,  for  it  tends  rapidly 
toward  stable  equilibrium,  and  the  crust  ought  therefore  to 
reach  a  state  of  repose  early  in  geologic  time.  But  there 
is  no  evidence  that  such  a  state  has  been  attained,  and  but 
little  if  any  evidence  of  diminished  activity  in  crustal  move- 
ments during  recent  geologic  time.  Hence  we  infer  that 
isostasy  is  competent  only  on  the  supposition  that  it  is 
kept  in  action  by  some  other  cause  tending  constantly 
to  disturb  the  equilibrium  which  would  otherwise  result. 
Such  a  cause  is  found  in  secular  contraction,  and  it  is  not 
improbable  that  these  two  seemingly  divergent  theories  are 
really  supplementary. 

Closely  related  to  the  questions  of  secular  contraction 
and  the  mechanics  of  crust  movements  are  those  vexed 
questions  of  earthquakes,  volcanism,  the  liquidity  or  solid- 
ity of  the  interior,  and  the  rigidity  of  the  earth's  mass  as 

30  Dutton,  Captain  C.  E.,  "  On  some  of  the  Greater  Problem's  of 
Physical  Geology,"  "  Bulletin  Philosophical  Society  of  Washington," 
vol.  xi,  pp.  51-64. 


MATHEMATICAL  THEORIES  OF  THE  EARTH 


125 


a  whole;  all  questions  of  the  greatest  interest,  but  still  lin- 
gering on  the  battlefields  of  scientific  opinion.  Many  of 
the  "  thrice-slain  "  combatants  in  these  contests  would  fain 
risk  being  slain  again;  and  whether  our  foundation  be 
liquid  or  solid,  or,  to  speak  more  precisely,  whether  the 
earth  may  not  be  at  once  highly  plastic  under  the  action  of 
long-continued  forces  and  highly  rigid  under  the  action 
of  periodic  forces  of  short  period,  it  is  pretty  certain  that 
some  years  must  elapse  before  the  arguments  will  be  con- 
vincing to  all  concerned.  The  difficulties  appear  to  be  due 
principally  to  our  profound  ignorance  of  the  properties  of 
matter  subject  to  the  joint  action  of  great  pressure  and 
great  heat.  The  conditions  which  exist  a  few  miles  be- 
neath the  surface  of  the  earth  are  quite  beyond  the  reach  of 
laboratory  tests  as  hitherto  developed,  but  it  is  not  clear 
how  our  knowledge  is  to  be  improved  without  resort  to 
experiments  of  a  scale  in  some  degree  comparable  with 
the  facts  to  be  explained.  In  the  mean  time,  therefore,  we 
may  expect  to  go  on  theorizing,  adding  to  the  long  list  of 
dead  theories  which  mark  the  progress  of  scientific  thought 
with  the  hope  of  attaining  the  truth  not  so  much  by  di- 
rect discovery  as  by  the  laborious  process  of  eliminating 
error. 

When  we  take  a  more  comprehensive  view  of  the  prob- 
lems presented  by  the  earth,  and  look  for  light  on  their 
solution  in  theories  of  cosmogony,  the  difficulties  which 
beset  us  are  no  less  numerous  and  formidable  than  those 
encountered  along  special  lines  of  attack.  Much  progress 
has  recently  been  made,  however,  in  the  elaboration  of  such 
theories.  Roche,31  Darwin,32  and  others  have  done  much 
to  remove  the  nebulosity  of  Laplace's  nebular  hypothesis. 

81  "  Essai  sur  la  Constitution  et  1'origine  du  systeme  solaire,"  par 
M.  Edouard  Roche.    "  Memoires  de  1' Academic  des  Sciences  et  Lettres 
de  Montpellier,"  tome  viii,  1873. 

82  "  On  the  Precession  of  a  Viscous  Spheroid  and  on  the  Remote 
History  of  the  Earth,"  "  Phil.  Trans.,"  part  ii,  1879.    "  On  the  Secular 
Changes  in  the  Elements  of  the  Orbit  of  a  Satellite  revolving  about  a 
Tidally  Distorted  Planet,"   "Phil.  Trans.,"  part  ii,    1880.     "On  the 
Tidal  Friction  of  a  Planet  attended  by  Several  Satellites,  and  on  the 
Evolution  of  the  Solar  System,"  "  Phil.  Trans.,"  part  ii,  1881. 


126  WOODWARD 

Poincare  83  and  Darwin  84  have  gone  far  toward  bridging 
the  gaps  which  have  long  rendered  the  theory  of  rotating 
fluid  masses  incomplete.  Poincare  has,  in  fact,  shown  us 
how  a  homogeneous  rotating  mass  might,  through  loss  of 
heat  and  consequent  contraction,  pass  from  the  spheroidal 
form  to  the  Jacobian  ellipsoidal  form,  and  thence,  by  rea- 
son of  its  increasing  speed  of  rotation,  separate  into  two  un- 
equal masses.  Darwin,  starting  with  a  swarm  of  meteorites 
and  gravitation  as  a  basis,  has  reached  many  interesting  and 
instructive  results  in  the  endeavour  to  trace  out  the  laws  of 
evolution  of  a  planetary  system.35  But  notwithstanding 
the  splendid  researches  of  these  and  other  investigators  in 
this  field,  it  must  be  said  that  the  real  case  of  the  solar 
system,  or  of  the  earth  and  moon,  still  defies  analysis;  and 
that  the  mechanics  of  the  segregation  of  a  planet  from  the 
sun,  or  of  a  satellite  from  a  planet,  if  such  an  event  has 
ever  happened,  or  the  mechanics  of  the  evolution  of  a  solar 
system  from  a  swarm  of  meteorites,  are  still  far  from  being 
clearly  made  out. 

Time  does  not  permit  me  to  make  anything  but  the 
briefest  allusion  to  the  comparatively  new  science  of  mathe- 
matical meteorology  with  its  already  considerable  list  of 
well-defined  theories  pressing  for  acceptance  or  rejection. 
Nor  need  I  say  more  with  reference  to  those  older  mathe- 
matical questions  of  the  tides  and  terrestrial  magnetism  than 
that  they  are  still  unsettled.  These  and  many  other  ques- 
tions, old  and  new,  might  serve  equally  well  to  illustrate  the 
principal  fact  that  this  address  has  been  designed  to  em- 
phasize, namely,  that  the  mathematical  theories  of  the 
earth  already  advanced  and  elaborated  are  by  no  means 
complete,  and  that  no  mathematical  Alexander  need  yet 
pine  for  other  worlds  to  conquer. 

Speculations  concerning  the  course  and  progress  of  sci- 

83  Sur  1'equilibre  d'une  masse  fluide  animee  d'un  mouvement  de  rota- 
tion. ("Acta  Mathematical  vol.  vii,  1885.) 

"  On  Figures  of  Equilibrium  of  Rotating  Masses  of  Fluid,"  "  Phil. 
Trans./'  vol.  clxxviii,  1887. 

"  On  the  Mechanical  Conditions  of  a  Swarm  of  Meteorites  and 
on  Theories  of  Cosmogony,"  "  Phil.  Trans.,"  vol.  clxxx,  1889. 


MATHEMATICAL  THEORIES  OF  THE  EARTH       127 

ence  are  usually  untrustworthy  if  not  altogether  fallacious. 
But,  being  delegated  for  the  hour  to  speak  to  and  for 
mathematicians  and  astronomers,  it  may  be  permissible  to 
offer,  in  closing,  a  single  suggestion,  which  will  perhaps 
help  us  to  orient  ourselves  aright  in  our  various  fields  of 
research.  If  the  curve  of  scientific  progress  in  any  domain 
of  thought  could  be  drawn,  there  is  every  reason  to  believe 
that  it  would  exhibit  considerable  irregularities.  There 
would  be  marked  maxima  and  minima  in  its  general  tend- 
ency toward  the  limit  of  perfect  knowledge;  and  it  seems 
not  improbable  that  the  curve  would  show  throughout  some 
portions  of  its  length  a  more  or  less  definitely  periodic  suc- 
cession of  maxima  and  minima.  Races  and  communities 
as  well  as  individuals,  the  armies  in  pursuit  of  truth  as  well 
as  those  in  pursuit  of  plunder,  have  their  periods  of  cul- 
minating activity  and  their  periods  of  placid  repose.  It  is 
a  curious  fact  that  the  history  of  the  mathematical  theories 
of  the  earth  presents  some  such  periodicity.  We  have  the 
marked  maximum  of  the  epoch  of  Newton  near  the  end 
of  the  seventeenth  century,  with  the  equally  marked  maxi- 
mum of  the  epoch  of  Laplace  near  the  end  of  the  eighteenth 
cntury;  and,  judging  from  the  recent  revival  of  geodesy 
and  astronomy  in  Europe,  and  from  the  well-nigh  general 
activity  in  mathematical  and  geological  research,  we  may 
hope,  if  not  expect,  that  the  end  of  the  present  century  will 
signalize  a  similar  epoch  of  productive  activity.  The 
minima  periods  which  followed  the  epochs  of  Newton  and 
Laplace  are  less  definitely  marked  but  not  less  noteworthy 
and  instructive.  They  were  not  periods  of  placid  repose; 
to  find  such  one  must  go  back  into  the  night  of  the  middle 
ages;  but  they  were  periods  of  greatly  diminished  energy, 
periods  during  which  those  who  kept  alive  the  spirit  of  in- 
vestigation were  almost  as  conspicuous  for  their  isolation 
as  for  their  distinguished  abilities.  Many  causes,  of  course, 
contributed  to  produce  these  minima  periods,  and  it  would 
be  an  interesting  study  in  philosophic  history  to  trace  out 
the  tendency  and  effect  of  each  cause.  It  is  desired  here, 
however,  to  call  attention  to  only  one  cause  which  con- 


128  WOODWARD 

tributed  to  the  somewhat  general  apathy  of  the  periods 
mentioned,  and  which  always  threatens  to  dampen  the 
ardour  of  research  immediately  after  the  attainment  of  any 
marked  success  or  advance.  I  refer  to  the  impression  of 
contentment  with  and  acquiescence  in  the  results  of  science, 
which  seems  to  find  easy  access  to  trained  as  well  as  un- 
trained minds  before  an  investigation  is  half  completed  or 
even  fairly  begun.  That  some  such  tacit  persuasion  of  the 
completeness  of  the  knowledge  of  the  earth  has  at  times 
pervaded  scientific  thought,  there  can  be  no  doubt.  This 
was  notably  the  case  during  the  period  which  followed  the 
remarkable  epoch  of  Laplace.  The  profound  impression 
of  the  sufficiency  of  the  brilliant  discoveries  and  advances 
of  that  epoch  is  aptly  described  by  Carlyle  in  the  half 
humorous,  half  sarcastic  language  of  Sartor  Resartus. 
"  Our  theory  of  gravitation,"  he  says,  "  is  as  good  as  per- 
fect: Lagrange,  it  is  well  known,  has  proved  that  the  plan- 
etary system,  on  this  scheme,  will  endure  forever;  Laplace, 
still  more  cunningly,  even  guesses  that  it  could  hot  have 
been  made  on  any  other  scheme.  Whereby,  at  least,  our 
nautical  log-books  can  be  better  kept;  and  water  transport 
of  all  kinds  has  grown  more  commodious.  Of  geology  and 
geognosy  we  know  enough;  what  with  the  labours  of  our 
Werners  and  Huttons,  what  with  the  ardent  genius  of  their 
disciples,  it  has  come  about  that  now,  to  many  a  royal  so- 
ciety, the  creation  of  a  world  is  little  more  mysterious  than 
the  cooking  of  a  dumpling;  concerning  which  last,  indeed, 
there  have  been  minds  to  whom  the  question — How  the 
apples  were  got  in — presented  difficulties."  This  was  writ- 
ten nearly  sixty  years  ago,  about  the  time  the  sage  of 
Ecclefechan  abandoned  his  mathematics  and  astronomy  for 
literature  to  become  the  sees  of  Chelsea;  but  the  force  of 
its  irony  is  still  applicable,  for  we  have  yet  to  learn,  essen- 
tially, "  How  the  apples  were  got  in,"  and  what  kind 
they  are. 

As  to  the  future,  we  can  only  guess,  less  or  more  vaguely, 
from  our  experience  in  the  past  and  from  our  knowledge  of 
present  needs.  Though  the  dawn  of  that  future  is  certainly 


MATHEMATICAL  THEORIES  OF  THE  EARTH      129 

not  heralded  by  rosy  tints  of  overconfidence  among  those 
acquainted  with  the  difficulties  to  be  overcome,  the  prospect, 
on  the  whole,  has  never  been  more  promising.  The  con- 
verging lights  of  many  lines  of  investigation  are  now 
brought  to  bear  on  the  problems  presented  by  our  planet. 
There  is  ample  reason  to  suppose  that  our  day  will  witness 
a  fair  average  of  those  happy  accidents  in  science  which 
lead  to  the  discovery  of  new  principles  and  new  methods. 
We  have  much  to  expect  from  the  elaborate  machinery  and 
perfected  methods  of  the  older  and  more  exact  sciences 
of  measuring  and  weighing — astronomy,  geodesy,  physics, 
and  chemistry.  We  have  more  to  expect,  perhaps,  from 
geology  and  meteorology,  with  their  vast  accumulation  of 
facts  not  yet  fully  correlated.  Much,  also,  may  be  antici- 
pated from  that  new  astronomy  which  looks  for  the  se- 
crets of  the  earth's  origin  and  history  in  nebulous  masses 
or  in  swarms  of  meteorites.  We  have  the  encouraging 
stimulus  of  a  very  general  and  rapidly  growing  popular 
concern  in  the  objects  of  our  inquiries,  and  the  freest 
avenues  for  the  dissemination  of  new  information;  so  that 
we  may  easily  gain  the  advantage  of  a  concentration  of 
energy  without  centralization  of  personal  interests.  To 
those,  therefore,  who  can  bring  the  prerequisites  of  endless 
patience  and  unflagging  industry,  who  can  bear  alike  the 
remorseless  discipline  of  repeated  failure  and  the  prosperity 
of  partial  success,  the  field  is  as  wide  and  as  inviting  as 
it  ever  was  to  a  Newton  or  a  Laplace. 


THE  ROTATION 

AND  PHYSICAL  CONSTITUTION 

OF  THE  PLANET  MERCURY 

AND 

THE  PLANET  MARS 

BY 

GIOVANNI   VIRGINIO   SCHIAPARELLI 


THE  ROTATION  AND  PHYSICAL  CONSTI- 
TUTION OF  THE  PLANET  MERCURY1 

A1ONG  the  older  planets  no  one  is  so  difficult  to 
observe  as  Mercury;  and  none  presents  so  many 
obstacles  to  the  investigation  of  its  orbit  as  well  as 
to  the  study  of  its  physical  nature.  With  respect  to  the 
orbit  it  is  enough  to  say  that  Mercury  is  the  only  planet 
whose  motions  it  has  been  declared  to  be  impossible  up 
to  the  present  time  to  subject  to  the  laws  of  universal 
gravitation;  and  the  theory  of  whose  orbit,  though  elab- 
orated by  the  sagacious  mind  of  a  Leverrier,  still  presents 
notable  discrepancies  with  observations.  As  to  its  phys- 
ical nature,  very  little  is  known,  and  of  that  little  it 
may  be  said  that  nearly  all  of  it  rests  upon  observations 
now  a  century  old,  made  at  Lilienthal  by  the  famous 
Schroeter. 

The  telescopic  examination  of  this  planet  is,  in  fact, 
most  difficult.  So  close  is  its  orbit  to  the  sun  that  Mer- 
cury never  appears  in  the  sky  far  enough  away  from  that 
great  luminary  to  admit  of  its  examination  in  complete 
darkness — at  least  not  in  our  latitudes.  Observations 
which  are  made  in  the  period  of  twilight,  before  the  rising 
or  after  the  setting  of  the  sun,  are  rarely  successful,  because 
under  such  circumstances  the  planet  is  always  near  to  the 
horizon,  and  so  subject  to  disturbances  and  unequal  re- 
fraction in  the  lowest  atmospheric  strata,  as  to  present  for 
the  most  part  in  the  telescope  that  uncertain  and  flaming 

XA  discourse  delivered  at  the  meeting  of  the  Royal  Academy  of 
the  Lincei,  December  8,  1889,  in  the  presence  of  the  King  and  Queen 
of  Italy.  Translated  by  SARA  CARR  UPTON. 

i33 


134 


SCHIAPARELLI 


aspect  which  strikes  the  naked  eye  as  a  bright  scintillation; 
for  that  very  reason  the  ancients  called  it  Stilbon,  which 
means  the  scintillating.  Observations  by  night  being 
therefore  impossible,  and  twilight  observations  being  rarely 
successful,  no  other  way  remains  but  to  make  the  attempt 
in  full  daylight,  in  the  continual  presence  of  the  sun,  and 
through  an  atmosphere  constantly  illuminated. 

Certain  trials  made  in  1881  persuaded  me  that  it 
would  be  possible  not  only  to  see  the  markings  on  Mer- 
cury in  full  daylight,  but  also  to  obtain  a  series  of  suf- 
ficiently connected  and  continuous  observations  of  these 
spots.  In  the  beginning  of  1882  I  determined  to  make  a 
regular  study  of  the  planet;  and  in  the  eight  following 
years  I  have  had  the  telescope  directed  upon  Mercury 
several  hundreds  of  times,  usually  to  little  purpose,  and 
with  the  loss  of  much  time;  sometimes  on  account  of  at- 
mospheric disturbance,  which  during  the  day  is  often  very 
great  (especially  in  the  summer  months);  and  again  on 
account  of  insufficient  transparency  of  the  air.  Neverthe- 
less, by  employing  the  necessary  patience,  I  succeeded  in 
seeing  the  spots  on  the  planet  with  greater  or  less  pre- 
cision on  more  than  one  hundred  and  fifty  occasions,  and 
also  in  making  at  such  times  some  rather  satisfactory  draw- 
ings. For  this  purpose  I  used  at  first  the  smaller  telescope 
of  our  observatory,  made  by  Merz,  which  was  often  found 
to  be  inadequate  for  observations  so  difficult  as  these.  But 
in  the  mean  time  the  new  great  equatorial  refractor  had 
been  installed  in  the  observatory  of  Milan.  It  may  be 
called  the  most  perfect  work  which  has  yet  come  from 
the  workshops  of  Munich.  By  its  aid  I  was  enabled  to 
pursue  the  work  with  greater  success,  and  to  attain  more 
complete  and  more  certain  results.  And  in  regard  to  this 
refractor  I  can  not  recall  without  lively  emotions  of  grati- 
tude the  warm  interest  shown  by  your  Majesties,  now 
eleven  years  ago,  when  it  was  a  question  of  providing  that 
noble  instrument  for  the  Milan  Observatory.  Nor  is  it 
possible  for  me  to  forget  the  generous  eagerness  with  which 
this  academy,  and  Quintius  Sella  of  glorious  memory  at 


THE   PLANET  MERCURY  135 

the  head  of  it,  supported  the  proposition  with  an  authorita- 
tive vote,  and  the  large  majority  by  which  it  was  honoured 
in  both  branches  of  Parliament.  The  new  facts  concern- 
ing the  planet  Mercury  which  this  telescope  has  revealed, 
I  consider  as  the  most  important  and  most  precious  results 
which  have  been  so  far  obtained  by  its  aid;  so  that  to  give 
the  first  account  of  these  new  things  at  this  time  and  in 
this  place  seems  to  me  the  fulfilment  of  a  duty. 

I  will  first  speak  of  the  rotation  of  the  planet,  which  I 
have  found  to  be  very  different  from  what  has  been  be- 
lieved up  to  the  present  time,  on  the  faith  of  the  few  and 
insufficient  observations  made  a  hundred  years  ago  with 
imperfect  telescopes.  The  manner  and  chief  peculiarities 
of  this  rotation,  which  it  has  taken  me  many  years  of  ob- 
servation to  establish,  may  be  described  in  few  words,  by 
saying  that  Mercury  revolves  around  the  sun  in  a  manner 
similar  to  that  in  which  the  moon  revolves  around  the 
earth.  As  the  moon  describes  its  orbit  around  the  earth, 
showing  to  us  always  very  nearly  the  same  face  and  the 
same  spots,  so  Mercury  in  its  orbit  around  the  sun  con- 
stantly presents  to  that  great  luminary  very  nearly  the  same 
hemisphere  of  its  surface. 

I  have  said  almost  the  same  hemisphere,  and  not  exactly 
the  same  hemisphere.  Mercury,  in  fact,  like  the  moon, 
presents  the  phenomenon  of  libration.  Observing  the  full 
moon  with  a  small  telescope  at  very  different  epochs,  we 
shall  find  that,  in  general,  the  same  spots  occupy  the  cen- 
tral region  of  its  disk;  but,  studying  more  minutely  these 
central  spots,  and  the  relations  of  their  distances  from  the 
eastern  border  of  the  moon,  and  from  the  western  border, 
we  shall  soon  ascertain  (as  did  Galileo,  now  two  hundred  and 
fifty  years  ago,  for  the  first  time)  that  they  oscillate  by  sen- 
sible amounts,  now  toward  the  right  hand  and  again  toward 
the  left.  This  phenomenon  is  named  the  libration  in  longi- 
tude, and  arises  chiefly  because  the  point  toward  which  the 
moon  perpetually  and  almost  exactly  2  directs  one  of  its 

'That  is,  taking  no  account  of  the  slight  inclination  of  the  lunar 
equator   with  respect  to  the  plane   of  its   orbit,   and   supposing  the 


136  SCHIAPARELLI 

diameters  is  not  the  centre  of  the  earth,  neither  is  it  the 
centre  of  the  lunar  elliptical  orbit,  but  that  one  of  the  two 
foci  of  that  orbit  which  is  not  occupied  by  the  earth.  This 
point  is  called  by  astronomers  the  upper  focus.  To  any  one 
stationed  at  this  point  the  moon  would  therefore  show 
always  the  same  aspect.  To  us,  who  are,  instead,  on  the 
average  forty-two  thousand  kilometres  distant  from  the 
point,  the  moon  shows  itself  in  slightly  different  aspects  at 
different  times,  turning  toward  us  now  more  of  its  eastern 
regions,  now  more  of  its  western. 

Exactly  similar  is  the  way  in  which  Mercury  presents 
itself  to  the  sun  during  the  various  phases  of  its  revolution 
about  that  body.  One  of  the  diameters  of  the  planet  is 
constantly  directed  not  toward  that  focus  of  its  elliptical 
orbit  which  is  occupied  by  the  sun,  but  toward  the  other 
focus — toward  the  upper  focus.  Now,  these  two  foci  being 
distant  from  each  other  not  less  than  one  fifth  of  the  whole 
diameter  of  the  orbit  of  Mercury,  the  libration  of  the  planet 
is  very  great;  and  that  point  of  Mercury  which  receives 
the  solar  rays  vertically  is  projected  on  the  surface  of  the 
planet,  and  oscillates  along  its  equator  in  an  arc  which  has 
an  amplitude  of  forty-seven  degrees — that  is,  more  than 
an  eighth  of  the  whole  circumference  of  the  equator  itself. 
The  complete  period  of  going  and  returning  is  equal  to  the 
time  employed  by  Mercury  in  moving  around  its  orbit — 
that  is,  almost  eighty-eight  terrestrial  days.  Mercury,  there- 
fore, is  continually  oriented  to  the  sun  as  a  magnetic  needle 
to  a  piece  of  iron;  but  it  does  not  point  thither  so  con- 
stantly as  not  to  permit  of  a  certain  oscillatory  motion  of 
the  planet  eastward  and  westward,  similar  to  the  moon's 
oscillation  with  respect  to  the  earth. 

This  oscillatory  motion  is  of  the  greatest  importance  in 
respect  to  the  physical  condition  of  the  planet.  Let  us  sup- 
pose for  a  moment  that  this  motion  did  not  exist  in  fact, 
and  that  Mercury  always  presented  the  same  hemisphere 

moon's  motion  in  this  orbit  to  be  the  so-called  simple  elliptic  motion, 
in  which  the  perturbations  of  the  true  anomaly  are  disregarded  as  well 
as  certain  terms  that  are  of  the  order  of  the  square  of  the  eccentricity. 


THE   PLANET   MERCURY  137 

to  the  light  and  heat  of  the  sun,  the  other  hemisphere  re- 
maining wrapped  in  perpetual  night.  That  point  of  the 
planet's  surface  which  lies  at  the  central  pole  of  the  illu- 
minated hemisphere  would  eternally  have  the  sun  vertically 
above  it;  all  other  places  upon  Mercury  which  are  reached 
by  the  sun's  rays  would  always  see  the  sun  above  the  same 
point  of  the  horizon,  at  the  same  altitude,  without  any 
apparent  motion  or  sensible  change  of  place  whatever. 
Therefore  such  places  would  have  no  alternation  of  day 
and  night,  and  no  vicissitude  of  seasons. 

Remaining  thus  perpetually  in  presence  of  the  sun, 
with  the  stars  always  invisible,  Mercury  having  no  moon, 
it  is  difficult  to  understand  how  the  dwellers  in  the  regions 
of  perpetual  day  could  make  any  determinations  of  time 
(or  have  any  chronology). 

Upon  Mercury  things  are  nearly  in  this  case,  but  not 
entirely  so.  The  oscillating  motion  to  which  we  have  seen 
the  body  of  the  planet  is  subjected  with  respect  to  the  sun 
would  be  attributed  by  an  observer  on  the  planet  to  the 
sun  itself,  just  as  we  are  used  to  attribute  to  the  sun  the 
diurnal  motion  of  rotation  which  actually  belongs  to  the 
earth.  While,  therefore,  to  us  the  sun  seems  to  revolve 
continually  from  east  to  west,  and  thus  determines  the 
period  of  night  and  day  to  be  twenty-four  hours,  an  ob- 
server on  Mercury  would  see  the  sun  describe  an  arc  of 
forty-seven  degrees,  with  an  alternating  motion  to  and  fro, 
upon  the  celestial  vault;  and  this  arc  would  remain  always 
in  the  same  position  with  respect  to  the  horizon  of  the 
observer.  A  complete  cycle  of  such  double  oscillations 
of  the  sun  would  last  almost  exactly  eighty-eight  terrestrial 
days;  and  according  as  the  arc  of  the  solar  oscillatory 
motion  aforesaid  is  all  above  the  spectator's  horizon,  or 
all  below  that  horizon,  or  partly  above  and  partly  below  it, 
there  would  be  different  appearances  and  a  different  dis- 
tribution of  heat  and  of  light.  Accordingly,  in  those  re- 
gions of  Mercury  where  the  arc  of  solar  oscillation  remains 
entirely  below  the  local  horizon,  the  sun  will  never  be 
seen,  and  there  will  be  continual  darkness.  In  such  re- 


1 38  SCHIAPARELLI 

gions,  which  occupy  nearly  three  eighths  of  all  the  planet, 
the  dense  and  eternal  night  can  never  be  abated  except 
by  occasional  sources  of  light,  such  as  refraction  and 
atmospheric  twilights,  by  polar  auroras,  or  similar  phe- 
nomena, to  which  may  be  added  the  faint  light  afforded  by 
the  stars  and  planets.  Another  region  of  Mercury  which 
also  comprises  three  eighths  of  the  whole  surface  will  have 
the  entire  arc  of  solar  oscillation  above  the  horizon,  and 
it  will  be  continually  exposed  to  the  rays  of  the  sun,  with- 
out any  variation  other  than  that  of  their  greater  or  less 
obliquity  during  the  various  phases  of  the  period  of  eighty- 
eight  days:  for  such  a  region  no  night  will  be  possible. 
And,  lastly,  there  are  other  regions,  comprising  in  all  a 
fourth  part  of  the  whole  planet,  for  which  the  arc  of  the 
apparent  oscillation  of  the  sun  is  in  part  above  the  horizon, 
and  part  below.  For  these  places  alone  alternations  of 
light  and  darkness  will  be  possible.  In  these  privileged 
regions  the  entire  period  of  eighty-eight  terrestrial  days 
will  be  divided  into  two  intervals:  one  all  light,  the  other 
all  darkness;  the  duration  of  each  will  be  equal  at  certain 
places;  in  others,  instead,  light  or  darkness  will  prevail  in 
greater  or  less  degree,  according  to  the  position  of  the 
place  upon  the  surface  of  Mercury,  and  according  as  a 
larger  or  smaller  portion  of  the  arc  before  described  re- 
mains above  its  horizon. 

Upon  a  planet  where  affairs  are  so  ordered  the  possi- 
bility of  organic  life  depends  upon  the  existence  of  an 
atmosphere  capable  of  distributing  the  solar  heat  over  dif- 
ferent regions  so  as  to  modify  the  extraordinary  excesses 
of  heat  and  of  cold.  The  presence  of  such  an  atmosphere 
upon  Mercury  was  conjectured  by  Schroeter  a  century  ago; 
in  my  observations  I  find  more  evident  indications  of  it, 
which  concur  in  making  the  probability  of  its  existence 
almost  a  certainty. 

One  of  the  first  proofs  springs  from  the  continually 
observed  fact  that  the  markings  on  the  planet,  visible  for 
the  most  part  when  they  are  found  in  the  central  regions 
of  the  disk,  are  less  visible  or  even  disappear  as  they 


THE  PLANET  MERCURY  139 

approach  its  circular  borders.  I  have  been  able  to  con- 
vince myself  that  this  does  not  occur  simply  from  the 
greater  obliquity  of  the  line  of  sight  due  to  perspective, 
but  from  the  fact  that  in  that  perimetral  position  there  is 
actually  a  greater  hindrance  to  the  vision,  and  this  seems 
to  be  due  to  nothing  else  but  the  greater  length  of  the 
path  which  the  visual  rays  coming  from  the  non-central 
spot  must  pass  through  in  the  atmosphere  of  Mercury  to 
reach  the  eye.  And  from  this  I  conclude  that  the  atmos- 
phere of  Mercury  is  less  transparent  than  that  of  Mars, 
and  that  it  more  nearly  resembles,  in  that  regard,  that  of 
the  earth. 

In  addition  to  this,  the  circular  border  of  the  planet, 
where  the  spots  become  less  visible,  always  appears  more 
luminous  than  the  rest  of  the  disk.  It  is  often  irregularly 
luminous,  in  certain  parts  more  so,  in  others  less;  and 
sometimes  along  this  edge  rather  brilliant  white  areas  may 
be  seen  which  remain  visible  for  several  consecutive  days, 
but  which  nevertheless  ace,  in  general,  changeable,  now  in 
one  portion  and  now  in  another.  I  attribute  this  fact  to 
condensations  in  the  interior  of  the  atmosphere  of  Mer- 
cury, which  must  reflect  the  solar  rays  outward  toward 
celestial  space,  and  more  strongly  as  they  become  more 
opaque.  Such  white  areas  are  also  often  seen  in  the  more 
central  parts  of  the  disk,  but  in  that  case  they  are  not  so 
brilliant  as  upon  the  border. 

But  there  is  still  more.  The  dark  spots  of  the  planet, 
although  permanent  in  form  and  arrangement,  are  not 
always  equally  apparent,  but  are  sometimes  more  intense 
and  sometimes  more  faint;  and  it  also  happens  that  some 
of  these  markings  occasionally  become  entirely  invisible. 
This  I  can  not  attribute  to  any  more  obvious  cause  than  to 
atmospheric  condensations  similar  to  our  terrestrial  clouds, 
which  prevent  more  or  less  completely  any  view  of  the  true 
surface  of  Mercury  in  any  portion.  The  clouded  regions  of 
the  earth  must  present  an  absolutely  identical  appearance 
to  a  person  viewing  them  from  the  depths  of  celestial  space. 

Concerning  the  nature  of  the  surface  of  Mercury  very 


140 


SCHIAPARELLI 


little  can  be  ascertained  from  the  observations  so  far  made. 
Thus  we  have  to  note  that  three  eighths  of  its  surface  re- 
main inaccessible  to  the  solar  rays,  and  hence  to  our  vision 
also;  and  there  is  very  little  hope  of  ever  knowing  anything 
about  it  with  certainty.  But,  nevertheless,  it  will  be  easy 
to  reach  precise  and  certain  knowledge  of  the  portion 
visible  to  us.  The  dark  spots,  even  when  they  are  not  ob- 
scured by  atmospheric  condensation  in  the  manner  men- 
tioned above,  appear  always  under  the  form  of  bands  of 
extremely  light  shadings,  which  under  ordinary  circum- 
stances can  only  be  observed  with  much  difficulty  and  great 
attention.  Upon  more  favourable  occasions  these  shadings 
have  a  warm  brown  tint  like  sepia,  which  nevertheless  is 
never  greatly  different  from  the  general  colour  of  the 
planet.  This  is  usually  of  a  light  rose  tint,  tending  toward 
a  copper  colour.  It  is  most  difficult  to  give  a  satisfactory 
graphic  representation  of  such  vague  and  diffused  forms 
or  bands,  specially  from  the  want  of  fixity  of  the  edges 
which  always  leaves  room  for  a  certain  choice.  Such  inde- 
terminate edges,  however,  I  have  reason  to  believe,  in 
most  cases  are  mere  appearances  arising  from  insufficient 
optical  power  of  the  instrument;  because  the  more  beau- 
tiful is  the  image  and  the  more  perfect  the  vision,  the  more 
manifest  is  the  tendency  of  the  shadings  to  dissolve  into  a 
number  of  minute  particles.  And  there  is  no  doubt  that 
by  using  a  more  powerful  telescope  all  would  appear  re- 
solved into  minuter  forms;  exactly  as  with  a  simple  opera- 
glass  we  may  see  those  irregular  and  indistinct  masses  of 
shading  which  every  one  can  see  with  the  naked  eye  upon 
the  moon  resolved  into  much  smaller  parts. 

Considering  the  difficulty  of  making  a  proper  study  of 
the  dark  spots  of  Mercury,  it  is  not  easy  to  express  a  well- 
founded  opinion  on  their  nature.  They  might  simply  de- 
pend upon  the  diverse  material  and  structure  of  the  solid 
superficial  strata,  as  we  know  to  be  the  case  with  the  moon. 
But  if  any  one,  taking  into  account  the  fact  that  there  exists 
an  atmosphere  upon  Mercury  capable  of  condensation  and 
perhaps  also  of  precipitation,  should  hold  the  opinion  that 


THE   PLANET  MERCURY  141 

there  was  something  in  those  dark  spots  analogous  to  our 
seas,  I  do  not  think  that  a  conclusive  argument  to  the  con- 
trary could  be  advanced.  And  as  those  spots  are  not 
grouped  in  great  masses,  but  are  dispersed  about  in  areas 
and  zones  of  no  great  width,  much  ramified,  and  alternated 
with  clear  spaces  with  some  uniformity,  it  might  be  con- 
cluded that  no  vast  oceans  and  great  continents  exist  upon 
Mercury,  but  that  the  liquid  and  solid  areas  mingle  with 
each  other  in  reciprocal  ways  and  with  frequent  vicissitudes, 
thus  giving  rise  to  a  condition  of  things  very  different  from 
that  which  exists  upon  the  earth,  and  one  which  perhaps 
we  might  envy. 

At  any  rate,  we  have  in  the  case  of  Mercury  (as  in 
Mars),  a  world  which  is  sufficiently  diverse  from  our  own; 
which  receives  light  and  heat  from  the  sun,  not  only  in  a 
greater  amount  but  in  a  different  manner  than  the  earth; 
and  where  life,  if  so  be  life  exists  there,  finds  conditions  so 
different  from  those  to  which  we  are  accustomed  that  we 
can  scarcely  imagine  them.  The  perpetual  presence  of  the 
sun  almost  vertically  above  certain  regions,  and  its  per- 
petual absence  from  other  regions,  appears  to  us  to  be 
something  intolerable.  But  we  must  recollect  that  such  a 
contrast  should  produce  an  atmospheric  circulation  which 
is  at  the  same  time  stronger,  more  rapid,  and  more  regular 
than  that  which  sows  the  elements  of  life  on  the  earth;  and 
that  on  this  account  it  may  come  about  that  an  equilibrium 
of  temperature  is  produced  quite  as  complete  as  ours,  and 
possibly  even  more  so. 

Mercury  is  conspicuously  distinguished  from  the  other 
planets  by  the  manner  of  its  revolution  around  the  sun, 
turning  always  the  same  face  toward  it.  All  the  other 
planets  (at  least  so  far  as  is  ascertained  in  the  cases  which 
it  has  been  possible  to  determine)  revolve  rapidly  around 
their  axes  in  the  space  of  a  few  hours.  Mercury's  manner 
of  revolution,  however,  which  is  unique  among  the  planets, 
seems,  on  the  other  hand,  quite  usual  for  the  satellites; 
such  at  least  is  true  in  all  the  cases  where  it  has  been  pos- 
sible to  investigate  the  rotation  motion  of  a  satellite.  That 


I42  SCHIAPARELLI 

our  own  satellite  has  always  in  the  memory  of  man  shown 
to  the  earth  the  same  hemisphere,  is  certain  also  from  his- 
torical testimony,  because  Dante  speaks  of  "  Cain  and  the 
Thorns,"  and  among  the  smaller  works  of  Plutarch  there  is 
one  entitled  "  Of  the  Face  to  be  seen  in  the  Disk  of  the 
Moon."  That  the  satellites  of  Jupiter  show  always  the 
same  face  to  their  chief  planet  is  probable  as  to  the  first 
three,  and  for  the  fourth  it  is  absolutely  proved  by  the 
observations  of  Auwers  and  Engelmann.  The  same  fact 
had  been  discovered  by  William  Herschel  in  the  case  of 
Japetus,  the  eighth  and  most  distant  satellite  of  Saturn. 
That  which  would  seem  to  be  the  general  rule  for  the 
satellites  is  therefore,  as  exemplified  in  the  case  of  Mercury, 
the  exception  among  the  planets. 

Such  an  exception,  however,  seems  not  without  cause, 
and  it  is  probably  connected  with  the  fact  of  Mercury's 
great  proximity  to  the  sun,  and  perhaps  also  with  the  other 
fact  that  Mercury  is  without  satellites.  In  my  opinion,  it 
depends  also  upon  the  way  in  which  Mercury  was  gener- 
ated at  the  time  when  the  solar  system  took  its  present 
form.  The  singularity  of  Mercury  constitutes,  therefore,  a 
new  document  to  add  to  those  which  must  be  considered 
in  the  study  of  the  solar  and  planetary  cosmogony. 


THE   PLANET  MARS1 

MANY  of  the  first  astronomers  who  studied  Mars 
with  the  telescope  had  noted  on  the  outline  of  its 
disk  two  brilliant  white  spots  of  rounded  form  and 
of  variable  size.  In  process  of  time  it  was  observed  that 
while  the  ordinary  spots  upon  Mars  were  displaced  rapidly 
in  consequence  of  its  daily  rotation,  changing  in  a  few 
hours  both  their  position  and  their  perspective,  the  two 
white  spots  remained  sensibly  motionless  at  their  posts. 
It  was  concluded  rightly  from  this  that  they  must  oc- 
cupy the  poles  of  rotation  of  the  planet,  or  at  least  must 
be  found  very  near  to  them.  Consequently  they  were 
given  the  name  of  polar  caps  or  spots.  And  not  without 
reason  is  it  conjectured  that  these  represent  upon  Mars 
that  immense  mass  of  snow  and  ice  which  still  to-day  pre- 
vents navigators  from  reaching  the  poles  of  the  earth. 
We  are  led  to  this  conclusion  not  only  by  the  analogy 
of  aspect  and  of  place,  but  also  by  another  important 
observation. 

As  things  stand,  it  is  manifest  that  if  the  above-men- 
tioned white  polar  spots  of  Mars  represent  snow  and  ice 
they  should  continue  to  decrease  in  size  with  the  approach 
of  summer  in  those  places  and  increase  during  the  winter. 
Now  this  very  fact  is  observed  in  the  most  evident  man- 
ner. In  the  second  half  of  the  year  1892  the  southern 
polar  cap  was  in  full  view;  during  that  interval,  and  espe- 
cially in  the  months  of  July  and  August,  its  rapid  diminu- 
tion from  week  to  week  was  very  evident  to  those  observ- 

1  From  "  Natura  ed  Arte,"  February  15,  1893.  Translated  by  WILLIAM 
H.  PICKERING. 

143 


SCHIAPARELLI 

ing  with  common  telescopes.  This  snow  (for  we  may  well 
call  it  so),  which  in  the  beginning  reached  as  far  as  latitude 
70°,  and  formed  a  cap  of  over  2,000  kilometres  (1,200  miles) 
in  diameter,  progressively  diminished,  so  that  two  or  three 
months  later  little  more  of  it  remained  than  an  area  of  per- 
haps 300  kilometres  (180  miles)  at  the  most,  and  still  less 
was  seen  in  the  last  days  of  1892.  In  these  months  the 
southern  hemisphere  of  Mars  had  its  summer,  the  summer 
solstice  occurring  upon  October  I3th.  Correspondingly 
the  mass  of  snow  surrounding  the  northern  pole  should 
have  increased;  but  this  fact  was  not  observable,  since  that 
pole  was  situated  in  the  hemisphere  of  Mars  which  was 
opposite  to  that  facing  the  earth.  The  melting  of  the 
northern  snow  was  seen  in  its  turn  in  the  years  1882,  1884, 
and  1886. 

These  observations  of  the  alternate  increase  and  de- 
crease of  the  polar  snows  are  easily  made  even  with  tele- 
scopes of  moderate  power,  but  they  become  much  more 
interesting  and  instructive  when  we  can  follow  assidu- 
ously the  changes  in  their  more  minute  particulars,  using 
larger  instruments.  The  snowy  regions  are  then  seen 
to  be  successively  notched  at  their  edges;  black  holes 
and  huge  fissures  are  formed  in  their  interiors;  great  iso- 
lated pieces  many  miles  in  extent  stand  out  from  the  prin- 
cipal mass  and,  dissolving,  disappear  a  little  later.  In  short, 
the  same  divisions  and  movements  of  these  icy  fields  pre- 
sent themselves  to  us  at  a  glance  that  occur  during  the 
summer  of  our  own  arctic  regions,  according  to  the  de- 
scriptions of  explorers. 

The  southern  snow,  however,  presents  this  peculiarity: 
The  centre  of  its  irregularly  rounded  figure  does  not  coin- 
cide exactly  with  the"  pole,  but  is  situated  at  another  point, 
which  is  nearly  always  the  same,  and  is  distant  from  the 
pole  about  300  kilometres  (180  miles)  in  the  direction  of 
the  Mare  Erythraeum.  From  this  we  conclude  that  when 
the  area  of  the  snow  is  reduced  to  its  smallest  extent  the 
south  pole  of  Mars  is  uncovered,  and  therefore,  perhaps, 
the  problem  of  reaching  it  upon  this  planet  is  easier  than 


THE   PLANET   MARS  145 

upon  the  earth.  The  southern  snow  is  in  the  midst  of  a 
huge  dark  spot,  which  with  its  branches  occupies  nearly  one 
third  of  the  whole  surface  of  Mars,  and  is  supposed  to 
represent  its  principal  ocean.  Hence  the  analogy  with  our 
arctic  and  antarctic  snows  may  be  said  to  be  complete,  and 
especially  so  with  the  antarctic  one. 

The  mass  of  the  northern  snow  cap  of  Mars  is,  on  the 
other  hand,  centred  almost  exactly  upon  its  pole.  It  is 
located  in  a  region  of  yellow  colour,  which  we  are  accus- 
tomed to  consider  as  representing  the  continent  of  the 
planet.  From  this  arises  a  singular  phenomenon  which  has 
no  analogy  upon  the  earth.  At  the  melting  of  the  snows 
accumulated  at  that  pole  during  the  long  night  of  ten 
months  and  more  the  liquid  mass  produced  in  that  opera- 
tion is  diffused  around  the  circumference  of  the  snowy 
region,  converting  a  large  zone  of  surrounding  land  into  a 
temporary  sea  and  filling  all  the  lower  regions.  This  pro- 
duces a  gigantic  inundation,  which  has  led  some  observers 
to  suppose  the  existence  of  another  ocean  in  those  parts, 
but  which  does  not  really  exist  in  that  place,  at  least  as  a 
permanent  sea.  We  see  then  (the  last  opportunity  was 
in  1884)  the  white  spot  of  the  snow  surrounded  by  a  dark 
zone,  which  follows  its  perimeter  in  its  progressive  diminu- 
tion, upon  a  circumference  ever  more  and  more  narrow. 
The  outer  part  of  this  zone  branches  out  into  dark 
lines,  which  occupy  all  the  surrounding  region,  and  seem 
to  be  distributary  canals  by  which  the  liquid  mass  may 
return  to  its  natural  position.  This  produces  in  these  re- 
gions very  extensive  lakes,  such  as  that  designated  upon 
the  map  by  the  name  of  Lacus  Hyperboreus;  the  neigh- 
bouring interior  sea  called  Mare  Acidalium  becomes  more 
black  and  more  conspicuous.  And  it  is  to  be  remembered 
as  a  very  probable  thing  that  the  flowing  of  this  melted 
snow  is  the  cause  which  determines  principally  the  hydro- 
graphic  state  of  the  planet  and  the  variations  that  are  peri- 
odically observed  in  its  aspect.  Something  similar  would 
be  seen  upon  the  earth  if  one  of  our  poles  came  to  be 
located  suddenly  in  the  centre  of  Asia  or  of  Africa.  As 

10 


I46  SCHIAPARELLI 

things  stand  at  present,  we  may  find  a  miniature  image  of 
these  conditions  in  the  flooding  that  is  observed  in  our 
streams  at  the  melting  of  the  Alpine  snows. 

Travellers  in  the  arctic  regions  have  frequent  occasion 
to  observe  how  the  state  of  the  polar  ice  at  the  beginning 
of  the  summer,  and  even  at  the  beginning  of  July,  is  always 
very  unfavourable  to  their  progress.  The  best  season  for 
exploration  is  in  the  month  of  August,  and  September  is 
the  month  in  which  the  trouble  from  the  ice  is  the  least. 
Thus  in  September  our  Alps  are  usually  more  practicable 
than  at  any  other  season.  And  the  reason  for  it  is  clear — 
the  melting  of  the  snow  requires  time;  a  high  temperature 
is  not  sufficient;  it  is  necessary  that  it  should  continue, 
and  its  effect  will  be  so  much  the  greater,  as  it  is  the  more 
prolonged.  Thus,  if  we  could  slow  down  the  course  of  our 
season  so  that  each  month  should  last  sixty  days  instead 
of  thirty,  in  the  summer,  in  such  a  lengthened  condition, 
the  melting  of  the  ice  would  progress  much  further,  and 
perhaps  it  would  not  be  an  exaggeration  to  say  that  the 
polar  cap  at  the  end  of  the  warm  season  would  be  entirely 
destroyed.  But  one  can  not  doubt,  in  any  case,  that  the 
fixed  portion  of  such  a  cap  would  be  reduced  to  much 
smaller  size  than  we  see  it  to-day.  Now,  this  is  exactly 
what  happens  on  Mars.  The  long  year,  nearly  double  our 
own,  permits  the  ice  to  accumulate  during  the  polar  night 
of  ten  or  twelve  months,  so  as  to  descend  in  the  form  of 
a  continuous  layer  as  far  as  parallel  70°,  or  even  farther. 
But  in  the  day  which  follows,  of  twelve  or  ten  months,  the 
sun  has  time  to  melt  all,  or  nearly  all,  of  the  snow  of  recent 
formation,  reducing  it  to  such  a  small  area  that  it  seems  to 
us  no  more  than  a  very  white  point.  And  perhaps  this 
snow  is  entirely  destroyed;  but  of  this  there  is  at  present 
no  satisfactory  observation. 

Other  white  spots  of  a  transitory  character  and  of  a 
less  regular  arrangement  are  formed  in  the  southern  hemi- 
sphere upon  the  islands  near  the  pole,  and  also  in  the  oppo- 
site hemisphere  whitish  regions  appear  at  times  surround- 
ing the  north  pole  and  reaching  to  50°  and  55°  of  latitude. 


THE   PLANET  MARS  147 

They  are,  perhaps,  transitory  snows,  similar  to  those  which 
are  observed  in  our  latitudes.  But  also  in  the  torrid  zone  of 
Mars  are  seen  some  very  small  white  spots  more  or  less  per- 
sistent; among  others  one  was  seen  by  me  in  three  con- 
secutive oppositions  (1877-1882)  at  the  point  indicated 
upon  our  chart  by  longitude  268°  and  latitude  16°  north. 
Perhaps  we  may  be  permitted  to  imagine  in  this  place  the 
existence  of  a  mountain  capable  of  supporting  extensive  ice 
fields.  The  existence  of  such  a  mountain  has  also  been 
suggested  by  some  recent  observers  upon  other  grounds. 

As  has  been  stated,  the  polar  snows  of  Mars  prove  in 
an  incontrovertible  manner  that  this  planet,  like  the  earth, 
is  surrounded  by  an  atmosphere  capable  of  transporting 
vapour  from  one  place  to  another.  These  snows  are,  in 
fact,  precipitations  of  vapour,  condensed  by  the  cold  and 
carried  with  it  successively.  How  carried  with  it  if  not  by 
atmospheric  movement?  The  existence  of  an  atmosphere 
charged  with  vapour  has  been  confirmed  also  by  spectro- 
scopic  observations,  principally  those  of  Vogel,  according 
to  which  this  atmosphere  must  be  of  a  composition  differing 
little  from  our  own,  and,  above  all,  very  rich  in  aqueous  va- 
pour. This  is  a  fact  of  the  highest  importance,  because  from 
it  we  can  rightly  affirm  with  much  probability  that  to  water 
and  to  no  other  liquid  is  due  the  seas  of  Mars  and  its  polar 
snows.  When  this  conclusion  is  assured  beyond  all  doubt, 
another  one  may  be  derived  from  it  of  not  less  importance 
— that  the  temperature  of  the  Arean  climate,  notwithstand- 
ing the  greater  distance  of  that  planet  from  the  sun,  is  of 
the  same  order  as  the  temperature  of  the  terrestrial  one. 
Because,  if  it  were  true,  as  has  been  supposed  by  some 
investigators,  that  the  temperature  of  Mars  was  on  the 
average  very  low  (from  50°  to  60°  below  zero),  it  would 
not  be  possible  for  water  vapour  to  be  an  important  ele- 
ment in  the  atmosphere  of  that  planet,  nor  could  water 
be  an  important  factor  in  its  physical  changes,  but  would 
give  place  to  carbonic  acid,  or  to  some  other  liquid  whose 
freezing  point  was  much  lower. 

The  elements  of  the  meteorology  of  Mars  seem,  then, 


148  SCHIAPARELLI 

to  have  a  close  analogy  to  those  of  the  earth.  But  there 
are  not  lacking,  as  might  be  expected,  causes  of  dissimi- 
larity. From  circumstances  of  the  smallest  moment  Nature 
brings  forth  an  infinite  variety  in  its  operations.  Of  the 
greatest  influence  must  be  different  arrangement  of  the  seas 
and  the  continents  upon  Mars  and  upon  the  earth,  regard- 
ing which  a  glance  at  the  map  will  say  more  than  would 
be  possible  in  many  words.  We  have  already  emphasized 
the  fact  of  the  extraordinary  periodical  flood,  which  at 
every  revolution  of  Mars  inundates  the  northern  polar  re- 
gion at  the  melting  of  the  snow.  Let  us  now  add  that 
this  inundation  is  spread  out  to  a  great  distance  by  means 
of  a  network  of  canals,  perhaps  constituting  the  principal 
mechanism  (if  not  the  only  one)  by  which  water  (and  with 
it  organic  life)  may  be  diffused  over  the  arid  surface  of  the 
planet.  Because  on  Mars  it  rains  very  rarely,  or  perhaps 
even  it  does  not  rain  at  all.  And  this  is  the  proof. 

Let  us  carry  ourselves  in  imagination  into  celestial 
space,  to  a  point  so  distant  from  the  earth  that  we  may 
embrace  it  all  at  a  single  glance.  He  would  be  greatly  in 
error  who  had  expected  to  see  reproduced  there  upon  a 
great  scale  the  image  of  our  continents  with  their  gulfs  and 
islands  and  with  the  seas  that  surround  them  which  are 
seen  upon  our  artificial  globes.  Then  without  doubt  the 
known  forms  or  part  of  them  would  be  seen  to  appear 
under  a  vaporous  veil,  but  a  great  part  (perhaps  one  half) 
of  the  surface  would  be  rendered  invisible  by  the  immense 
fields  of  cloud,  continually  varying  in  density,  in  form,  and 
in  extent.  Such  a  hindrance,  most  frequent  and  continu- 
ous in  the  polar  regions,  would  still  impede  nearly  half  the 
time  the  view  of  the  temperate  zones,  distributing  itself  in 
capricious  and  ever-varying  configurations.  The  seas  of 
the  torrid  zone  would  be  seen  to  be  arranged  in  long 
parallel  layers,  corresponding  to  the  zone  of  equatorial  and 
tropical  calms.  For  an  observer  placed  upon  the  moon  the 
study  of  our  geography  would  not  be  so  simple  an  under- 
taking as  one  might  at  first  imagine. 

There  is  nothing  of  this  sort  in  Mars.    In  every  climate 


THE   PLANET   MARS  149 

and  under  every  zone  its  atmosphere  is  nearly  perpetually 
clear  and  sufficiently  transparent  to  permit  one  to  recog- 
nise at  any  moment  whatever  the  contours  of  the  seas  and 
continents  and,  more  than  that,  even  the  minor  configura- 
tions. Not,  indeed,  that  vapours  of  a  certain  degree  of 
opacity  are  lacking,  but  they  offer  very  little  impediment 
to  the  study  of  the  topography  of  the  planet.  Here  and 
there  we  see  appear  from  time  to  time  a  few  whitish  spots, 
changing  their  position  and  their  form,  rarely  extending 
over  a  very  wide  area.  They  frequent  by  preference  a  few 
regions,  such  as  the  islands  of  the  Mare  Australe,  and  on 
the  continents  the  regions  designated  on  the  map  with  the 
names  of  Elysium  and  Tempe.  Their  brilliancy  generally 
diminishes  and  disappears  at  the  meridian  hour  of  the  place, 
and  is  re-enforced  in  the  morning  and  evening  with  very 
marked  variations.  It  is  possible  that  they  may  be  layers 
of  cloud,  because  the  upper  portions  of  terrestrial  clouds 
where  they  are  illuminated  by  the  sun  appear  white.  But 
various  observations  lead  us  to  think  that  we  are  dealing 
rather  with  a  thin  veil  of  fog  instead  of  a  true  nimbus  cloud, 
carrying  storms  and  rain.  Indeed,  it  may  be  merely  a 
temporary  condensation  of  vapour  under  the  form  of  dew 
or  hoar  frost. 

Accordingly,  as  far  as  we  may  be  permitted  to  argue  from 
the  observed  facts,  the  climate  of  Mars  must  resemble  that 
of  a  clear  day  upon  a  high  mountain.  By  day  a  very  strong 
solar  radiation,  hardly  mitigated  at  all  by  mist  or  vapour; 
by  night  a  copious  radiation  from  the  soil  toward  celestial 
space,  and  because  of  that  a  very  marked  refrigeration. 
Hence  a  climate  of  extremes,  and  great  changes  of  tem- 
perature from  day  to  night,  and  from  one  season  to  an- 
other. And  as  on  the  earth  at  altitudes  of  5,000  and  6,000 
metres  (17,000  to  20,000  feet)  the  vapour  of  the  atmos- 
phere is  condensed  only  into  the  solid  form,  producing 
those  whitish  masses  of  suspended  crystals  which  we  call 
cirrus  clouds,  so  in  the  atmosphere  of  Mars  it  would  be 
rarely  possible  (or  would  even  be  impossible)  to  find  collec- 
tions of  cloud  capable  of  producing  rain  of  any  conse- 


150  SCHIAPARELLI 

quence.  The  variation  of  the  temperature  from  one  season 
to  another  would  be  notably  increased  by  their  long  dura- 
tion, and  thus  we  can  understand  the  great  freezing  and 
melting  of  the  snow,  which  is  renewed  in  turn  at  the  poles 
at  each  complete  revolution  of  the  planet  around  the  sun. 

As  our  chart  demonstrates,  in  its  general  topography 
Mars  does  not  present  any  analogy  with  the  earth.  A  third 
of  its  surface  is  occupied  by  the  great  Mare  Australe,  which 
is  strewn  with  many  islands,  and  the  continents  are  cut 
up  by  gulfs  and  ramifications  of  various  forms.  To  the  gen- 
eral water  system  belongs  an  entire  series  of  small  internal 
seas,  of  which  the  Hadriacum  and  the  Tyrrhenum  com- 
municate with  it  by  wide  mouths,  while  the  Cimmerium, 
the  Sirenum,  and  the  Solis  Lacus  are  connected  with  it 
only  by  means  of  narrow  canals.  We  shall  notice  in  the 
first  four  a  parallel  arrangement,  which  certainly  is  not 
accidental,  as  also  not  without  reason  is  the  corresponding 
position  of  the  peninsulas  of  Ausonia,  Hesperia,  and  Atlan- 
tis. The  colour  of  the  seas  of  Mars  is  generally  brown, 
mixed  with  gray,  but  not  always  of  equal  intensity  in  all 
places,  nor  is  it  the  same  in  the  same  place  at  all  times. 
From  an  absolute  black  it  may  descend  to  a  light  gray  or 
to  an  ash  colour.  Such  a  diversity  of  colours  may  have 
its  origin  in  various  causes,  and  is  not  without  analogy 
also  upon  the  earth,  where  it  is  noted  that  the  seas  of  the 
warm  zone  are  usually  much  darker  than  those  nearer  the 
pole.  The  water  of  the  Baltic,  for  example,  has  a  light, 
muddy  colour  that  is  not  observed  in  the  Mediterranean. 
And  thus  in  the  seas  of  Mars  we  see  the  colour  become 
darker  when  the  sun  approaches  their  zenith,  and  summer 
begins  to  rule  in  that  region. 

All  of  the  remainder  of  the  planet,  as  far  as  the  north 
pole,  is  occupied  by  the  mass  of  the  continents,  in  which, 
save  in  a  few  areas  of  relatively  small  extent,  an  orange 
colour  predominates,  which  sometimes  reaches  a  dark-red 
tint,  and  in  others  descends  to  yellow  and  white.  The 
variety  in  this  colouring  is  in  part  of  meteorological  origin, 
in  part  it  may  depend  on  the  diverse  nature  of  the  soil,  but 


THE   PLANET   MARS  151 

upon  its  real  cause  it  is  not  as  yet  possible  to  frame  any 
very  well  grounded  hypothesis.  Nevertheless,  the  cause  of 
this  predominance  of  the  red  and  yellow  tints  upon  the 
surface  of  ancient  Pyrois  is  well  known.2  Some  have 
thought  to  attribute  this  colouring  to  the  atmosphere  of 
Mars,  through  which  the  surface  of  the  planet  might  be 
seen  coloured,  as  any  terrestrial  object  becomes  red  when 
seen  through  red  glass.  But  many  facts  are  opposed  to 
this  idea,  among  others  that  the  polar  snows  appear  always 
of  the  purest  white,  although  the  rays  of  light  derived  from 
them  traverse  twice  the  atmosphere  of  Mars  under  great 
obliquity.  We  must  then  conclude  that  the  Arean  conti- 
nents appear  red  and  yellow  because  they  are  so  in  fact. 
Besides  these  dark  and  light  regions,  which  we  have 
described  as  seas  and  continents,  and  of  whose  nature  there 
is  at  present  scarcely  left  any  room  for  doubt,  some  others 
exist,  truly  of  small  extent,  of  an  amphibious  nature,  which 
sometimes  appear  yellowish  like  the  continents,  and  are 
sometimes  clothed  in  brown  (even  black  in  certain  cases), 
and  assume  the  appearance  of  seas,  while  in  other  cases 
their  colour  is  intermediate  in  tint,  and  leaves  us  in  doubt 
to  which  class  of  regions  they  may  belong.  Thus  all  the 
islands  scattered  through  the  Mare  Australe  and  the  Mare 
Erythraeum  belong  to  this  category;  so  too  the  long  pen- 
insula called  Deucalionis  Regio  and  Pyrrhse  Regio,  and  in 
the  vicinity  of  the  Mare  Acidalium  the  regions  designated 
by  the  names  of  Baltia  and  Nerigos.  The  most  natural 
idea,  and  the  one  to  which  we  should  be  led  by  analogy, 
is  to  suppose  these  regions  to  represent  huge  swamps,  in 
which  the  variation  in  depth  of  the  water  produces  the 
diversity  of  colours.  Yellow  would  predominate  in  those 
parts  where  the  depth  of  the  liquid  layer  was  reduced  to 
little  or  nothing,  and  brown,  more  or  less  dark,  in  those 
places  where  the  water  was  sufficiently  deep  to  absorb  more 
light  and  to  render  the  bottom  more  or  less  invisible.  That 
the  water  of  the  sea,  or  any  other  deep  and  transparent 

J  Pyrois  I  take  to  be  some  terrestrial  region,  although  I  have  not 
been  able  to  find  any  translation  of  the  name. — TRANSLATOR. 


I52  SCHIAPARELLI 

water,  seen  from  above,  appears  more  dark  the  greater 
the  depth  of  the  liquid  stratum,  and  that  the  land  in  com- 
parison with  it  appears  bright  under  the  solar  illumination, 
is  known  and  confirmed  by  certain  physical  reasons.  The 
traveller  in  the  Alps  often  has  occasion  to  convince  himself 
of  it,  seeing  from  the  summits  the  deep  lakes  with  which 
the  region  is  strewn  extending  under  his  feet  as  black  as 
ink,  while  in  contrast  with  them  even  the  blackest  rocks 
illumined  by  the  sunlight  appeared  brilliant.3 

Not  without  reason,  then,  have  we  hitherto  attributed 
to  the  dark  spots  of  Mars  the  part  of  seas,  and  that  of  con- 
tinents to  the  reddish  areas  which  occupy  nearly  two  thirds 
of  all  the  planet,  and  we  shall  find  later  other  reasons  which 
confirm  this  method  of  reasoning.  The  continents  form  in 
the  northern  hemisphere  a  nearly  continuous  mass,  the  only 
important  exception  being  the  great  lake  called  the  Mare 
Acidalium,  of  which  the  extent  may  vary  according  to  the 
time,  and  which  is  connected  in  some  way  with  the  inun- 
dations which  we  have  said  were  produced  by  the  melting 
of  the  snow  surrounding  the  north  pole.  To  the  system  of 
the  Mare  Acidalium  undoubtedly  belong  the  temporary 
lake  called  Lacus  Hyperboreus  and  the  Lacus  Niliacus. 
This  last  is  ordinarily  separated  from  the  Mare  Acidalium 
by  means  of  an  isthmus  or  regular  dam,  of  which  the  con- 
tinuity was  only  seen  to  be  broken  once  for  a  short  time 
in  1888.  Other  smaller  dark  spots  are  found  here  and  there 
in  the  continental  area  which  we  may  designate  as  lakes, 
but  they  are  certainly  not  permanent  lakes  like  ours,  but 
are  variable  in  appearance  and  size  according  to  the  sea- 
sons, to  the  point  of  wholly  disappearing  under  certain  cir- 
cumstances. Ismenius  Lacus,  Lunse  Lacus,  Trivium  Cha- 
rontis,  and  Propontis  are  the  most  conspicuous  and  durable 
ones.  There  are  also  smaller  ones,  such  as  Lacus  Moeris 

'This  observation  of  the  dark  colour  which  deep  water  exhibits 
when  seen  from  above  is  found  already  noted  by  the  first  author  of 
antique  memory,  for  in  the  "  Iliad  "  (verses  770,  771  of  book  v)  it  is  de- 
scribed how  "  the  sentinel  from  the  high  sentry  box  extends  his  glance 
over  the  wine-coloured  sea,  otvova  irAvrov"  In  the  version  of  Monti  the 
adjective  indicating  the  colour  is  lost. 


THE   PLANET  MARS  153 

and  Fons  Juventse,  which  at  their  maximum  size  do  not 
exceed  100  to  150  kilometres  (60  to  90  miles)  in  diameter, 
and  are  among  the  most  difficult  objects  upon  the  planet. 

All  the  vast  extent  of  the  continents  is  furrowed  upon 
every  side  by  a  network  of  numerous  lines  or  fine  stripes 
of  a  more  or  less  pronounced  dark  colour,  whose  aspect  is 
very  variable.  These  traverse  the  planet  for  long  distances 
in  regular  lines  that  do  not  at  all  resemble  the  winding 
courses  of  our  streams.  Some  of  the  shorter  ones  do  not 
reach  500  kilometres  (300  miles),  others,  on  the  other  hand, 
extend  for  many  thousands,  occupying  a  quarter  or  some- 
times even  a  third  of  a  circumference  of  the  planet.  Some 
of  these  are  very  easy  to  see,  especially  that  one  which  is 
near  the  extreme  left-hand  limit  of  our  map,  and  is  desig- 
nated by  the  name  of  Nilosyrtis.  Others  in  turn  are  ex- 
tremely difficult,  and  resemble  the  finest  thread  of  spider's 
web  drawn  across  the  disk.  They  are  subject  also  to  great 
variations  in  their  breadth,  which  may  reach  200  or  even 
300  kilometres  (120  to  180  miles)  for  the  Nilosyrtis,  while 
some  are  scarcely  30  kilometres  (18  miles)  broad. 

These  lines  or  stripes  are  the  famous  canals  of  Mars,  of 
which  so  much  has  been  said.  As  far  as  we  have  been 
able  to  observe  them  hitherto,  they  are  certainly  fixed  con- 
figurations upon  the  planet.  The  Nilosyrtis  has  been  seen 
in  that  place  for  nearly  one  hundred  years,  and  some  of 
the  others  for  at  least  thirty  years.  Their  length  and  ar- 
rangement are  constant,  or  vary  only  between  very  narrow 
limits.  Each  of  them  always  begins  and  ends  between  the 
same  regions.  But  their  appearance  and  their  degree  of 
visibility  vary  greatly,  for  all  of  them,  from  one  opposition 
to  another,  and  even  from  one  week  to  another,  and  these 
variations  do  not  take  place  simultaneously  and  according 
to  the  same  laws  for  all,  but  in  most  cases  happen  apparently 
capriciously,  or  at  least  according  to  laws  not  sufficiently 
simple  for  us  to  be  able  to  unravel.  Often  one  or  more  be- 
come indistinct,  or  even  wholly  invisible,  while  others  in 
their  vicinity  increase  to  the  point  of  becoming  conspicuous 
even  in  telescopes  of  moderate  power.  The  first  of  our 


154  SCHIAPARELLI 

maps  shows  all  those  that  have  been  seen  in  a  long  series  of 
observations.  This  does  not  at  all  correspond  to  the  appear- 
ance of  Mars  at  any  given  period,  because  generally  only 
a  few  are  visible  at  once.4 

Every  canal  (for  now  we  shall  so  call  them)  opens  at 
its  ends  either  into  a  sea,  or  into  a  lake,  or  into  another 
canal,  or  else  into  the  intersection  of  several  other  canals. 
None  of  them  have  yet  been  seen  cut  off  in  the  middle  of 
the  continent,  remaining  without  beginning  or  without  end. 
This  fact  is  of  the  highest  importance.  The  canals  may 
intersect  among  themselves  at  all  possible  angles,  but  by 
preference  they  converge  toward' the  small  spots  to  which 
we  have  given  the  name  of  lakes.  For  example,  seven  are 
seen  to  converge  in  Lacus  Phoenicis,  eight  in  Trivium 
Charontis,  six  in  Lunae  Lacus,  and  six  in  Ismenius  Lacus. 

The  normal  appearance  of  a  canal  is  that  of  a  nearly 
uniform  stripe,  black,  or  at  least  of  a  dark  colour,  similar  to 
that  of  the  seas,  in  which  the  regularity  of  its  general  course 
does  not  exclude  small  variations  in  its  breadth  and  small 
sinuosities  in  its  two  sides.  Often  it  happens  that  such  a 
dark  line  opening  out  upon  the  sea  is  enlarged  into  the 
form  of  a  trumpet,  forming  a  huge  bay,  similar  to  the 
estuaries  of  certain  terrestrial  streams.  The  Margaritifer 
Sinus,  the  Aonius  Sinus,  the  Auroras  Sinus,  and  the  two 
horns  of  the  Sabaeus  Sinus  are  thus  formed,  at  the  mouths 

*In  a  footnote  the  author  refers  to  a  drawing  of  Mars  made  by 
himself,  September  15,  1892,  and  says:  "...  At  the  top  of  the  disk 
the  Mare  Erythrseum  and  the  Mare  Australe  appear  divided  by  a  great 
curved  peninsula,  shaped  like  a  sickle,  producing  an  unusual  appear- 
ance in  the  area  called  Deucalionis  Regio,  which  was  prolonged  that 
year  so  as  to  reach  the  islands  of  Noachis  and  Argyre.  This  region 
forms  with  them  a  continuous  whole,  but  with  faint  traces  of  separa- 
tion occurring  here  and  there  in  a  length  of  nearly  six  thousand  kilo- 
metres (four  thousand  miles).  Its  colour,  much  less  brilliant  than  that 
of  the  continents,  was  a  mixture  of  their  yellow  with  the  brownish 
gray  of  the  neighbouring  seas."  The  interesting  feature  of  this  note 
is  the  remark  that  it. was  an  unusual  appearance,  the  region  referred 
to  being  that  in  which  the  central  branch  of  the  fork  of  the  Y  ap- 
peared. Since  no  such  branch  was  conspicuously  visible  this  year,  it 
would  therefore  seem  from  the  above  that  it  was  the  opposition  of  1892 
that  was  peculiar,  and  not  the  present  one. — TRANSLATOR. 


THE   PLANET  MARS  155 

of  one  or  more  canals,  opening  into  the  Mare  Erythraeum 
or  into  the  Mare  Australe.  The  largest  example  of  such  a 
gulf  is  the  Syrtis  Major,  formed  by  the  vast  mouth  of  the 
Nilosyrtis,  so  called.  This  gulf  is  not  less  that  1,800  kilo- 
metres (1,100  miles)  in  breadth,  and  attains  nearly  the  same 
depth  in  a  longitudinal  direction.  Its  surface  is  little  less 
than  that  of  the  Bay  of  Bengal.  In  this  case  we  see  clearly 
the  dark  surface  of  the  sea  continued  without  apparent  in- 
terruption into  that  of  the  canal.  Inasmuch  as  the  surfaces 
called  seas  are  truly  a  liquid  expanse,  we  can  not  doubt  that 
the  canals  are  a  simple  prolongation  of  them,  crossing  the 
yellow  areas  or  continents. 

Of  the  remainder,  that  the  lines  called  canals  are  truly 
great  furrows  or  depressions  in  the  surface  of  the  planet, 
destined  for  the  passage  of  the  liquid  mass  and  constituting 
for  it  a  true  hydrographic  system,  is  demonstrated  by  the 
phenomena  which  are  observed  during  the  melting  of  the 
northern  snows.  We  have  already  remarked  that  at  the 
time  of  melting  they  appeared  surrounded  by  a  dark  zone, 
forming  a  species  of  temporary  sea.  At  that  time  the 
canals  of  the  surrounding  region  become  blacker  and 
wider,  increasing  to  the  point  of  converting  at  a  certain 
time  all  of  the  yellow  region  comprised  between  the  edge 
of  the  snow  and  the  parallel  of  60°  north  latitude  into 
numerous  islands  of  small  extent.  Such  a  state  of  things 
does  not  cease  until  the  snow,  reduced  to  its  minimum  area, 
ceases  to  melt.  Then  the  breadth  of  the  canals  diminishes, 
the  temporary  sea  disappears,  and  the  yellow  region  again 
returns  to  its  former  area.  The  different  phases  of  these 
vast  phenomena  are  renewed  at  each  return  of  the  seasons, 
and  we  were  able  to  observe  them  in  all  their  particulars 
very  easily  during  the  oppositions  of  1882,  1884,  and  1886, 
when  the  planet  presented  its  northern  pole  to  terrestrial 
spectators.  The  most  natural  and  the  most  simple  inter- 
pretation is  that  to  which  we  have  referred,  of  a  great 
inundation  produced  by  the  melting  of  the  snows;  it  is 
entirely  logical  and  is  sustained  by  evident  analogy  with 
terrestrial  phenomena.  We  conclude,  therefore,  that  the 


1 56  SCHIAPARELLI 

canals  are  such  in  fact  and  not  only  in  name.  The  network 
formed  by  these  was  probably  determined  in  its  origin  in 
the  geological  state  of  the  planet,  and  has  come  to  be  slowly 
elaborated  in  the  course  of  centuries.  It  is  not  necessary 
to  suppose  them  the  work  of  intelligent  beings,  and,  not- 
withstanding the  almost  geometrical  appearance  of  all  of 
their  system,  we  are  now  inclined  to  believe  them  to  be 
produced  by  the  evolution  of  the  planet,  just  as  on  the 
earth  we  have  the  English  Channel  and  the  channel  of 
Mozambique. 

It  would  be  a  problem  not  less  curious  than  compli- 
cated and  difficult  to  study  the  system  of  this  immense 
stream  of  water,  upon  which  perhaps  depends  principally  the 
organic  life  upon  the  planet,  if  organic  life  is  found  there. 
The  variations  of  their  appearance  demonstrated  that  this 
system  is  not  constant.  When  they  become  displaced,  or 
their  outlines  become  doubtful  and  ill  defined,  it  is  fair  to 
suppose  that  the  water  is  getting  low  or  is  even  entirely 
dried  up.  Then,  in  place  of  the  canals  there  remains  either 
nothing  or  at  most  stripes  of  yellowish  colour  differing 
little  from  the  surrounding  background.  Sometimes  they 
take  on  a  nebulous  appearance,  for  which  at  present  it  is 
not  possible  to  assign  a  reason.  At  other  times  true  en- 
largements are  produced,  expanding  to  100,  200,  or  more 
kilometres  (60  to  120  miles)  in  breadth,  and  this  sometimes 
happens  for  canals  very  far  from  the  north  pole,  according 
to. laws  which  are  unknown.  This  occurred  in  Hydaspes 
in  1864,  in  Simois  in  1879,  in  Ackeron  in  1884,  and  in 
Triton  in  1888.  The  diligent  and  minute  study  of  the 
transformations  of  each  canal  may  lead  later  to  a  knowl- 
edge of  the  causes  of  these  effects. 

But  the  most  surprising  phenomenon  pertaining  to  the 
canals  of  Mars  is  their  gemination,  which  seems  to  occur 
principally  in  the  months  which  precede  and  in  those  which 
follow  the  great  northern  inundation — at  about  the  times 
of  the  equinoxes.  In  consequence  of  a  rapid  process,  which 
certainly  lasts  at  most  a  few  days,  or  even  perhaps  only  a 
few  hours,  and  of  which  it  has  not  yet  been  possible  to 


THE   PLANET  MARS  157 

determine  the  particulars  with  certainty,  a  given  canal 
changes  its  appearance  and  is  found  transformed  through 
all  its  length  into  two  lines  or  uniform  stripes  more  or  less 
parallel  to  one  another,  and  which  run  straight  and  equal 
with  the  exact  geometrical  precision  of  the  two  rails  of  a 
railroad.  But  this  exact  course  is  the  only  point  of  resem- 
blance with  the  rails,  because  in  dimensions  there  is  no  com- 
parison possible,  as  it  is  easy  to  imagine.  The  two  lines 
follow  very  nearly  the  direction  of  the  original  canal  and 
end  in  the  place  where  it  ended.  One  of  these  is  often 
superposed  as  exactly/  as  possible  upon  the  former  line, 
the  other  being  drawn  anew;  but  in  this  case  the  original 
line  loses  all  the  small  irregularities  and  curvature  that  it 
may  have  originally  possessed.  But  it  also  happens  that 
both  the  lines  may  occupy  opposite  sides  of  the  former 
canal  and  be  located  upon  entirely  new  ground.  The  dis- 
tance between  the  two  lines  differs  in  different  gemmations 
and  varies  from  600  kilometres  (360  miles)  and  more  down 
to  the  smallest  limit  at  which  two  lines  may  appear  sepa- 
rated in  large  visual  telescopes — less  than  an  interval  of 
50  kilometres  (30  miles).  The  breadth  of  the  stripes  them- 
selves may  range  from  the  limit  of  visibility,  which  we  may 
suppose  to  be  30  kilometres  (18  miles),  up  to  more  than 
loo  kilometres  (60  miles).  The  colour  of  the  two  lines 
varies  from  black  to  a  light  red,  which  can  hardly  be  dis- 
tinguished from  the  general  yellow  background  of  the  con- 
tinental surface.  The  space  between  is  for  the  most  part 
yellow,  but  in  many  cases  appears  whitish.  The  gemination 
is  not  necessarily  confined  only  to  the  canals,  but  tends  to 
be  produced  also  in  the  lakes.  Often  one  of  these  is  seen 
transformed  into  two  short,  broad  dark  lines  parallel  to  one 
another  and  traversed  by  a  yellow  line.  In  these  cases  the 
gemination  is  naturally  short  and  does  not  exceed  the  limits 
of  the  original  lake. 

The  gemination  is  not  shown  by  all  at  the  same  time, 
but  when  the  season  is  at  hand  it  begins  to  be  produced 
here  and  there,  in  an  isolated,  irregular  manner,  or  at  least 
without  any  easily  recognisable  order.  In  many  canals 


!58  SCHIAPARELLI 

(such  as  the  Nilosyrtis,  for  example)  the  gemination  is  lack- 
ing entirely,  or  is  scarcely  visible.  After  having  lasted  for 
some  months,  the  markings  fade  out  gradually  and  disap- 
pear until  another  season  equally  favourable  for  their  forma- 
tion. Thus  it  happens  that  in  certain  other  seasons  (espe- 
cially near  the  southern  solstice  of  the  planet)  few  are  seen, 
or  even  none  at  all.  In  different  oppositions  the  gemina- 
tion of  the  same  canal  may  present  different  appearances  as 
to  width,  intensity,  and  arrangement  of  the  two  stripes*; 
also  in  some  cases  the  direction  of  the  lines  may  vary, 
although  by  the  smallest  quantity,  but  still  deviating  by 
a  small  amount  from  the  canal  with  which  they  are  directly 
associated.  From  this  important  fact  it  is  immediately 
understood  that  the  gemination  can  not  be  a  fixed  forma- 
tion upon  the  surface  of  Mars  and  of  a  geographical  char- 
acter like  the  canals.  The  second  of  our  maps  will  give  an 
approximate  idea  of  the  appearance  which  these  singular 
formations  present.  It  contains  all  the  geminations  ob- 
served since  1882  up  to  the  present  time.  In  examining  it 
it  is  necessary  to  bear  in  mind  that  not  all  of  these  appear- 
ances were  simultaneous,  and  consequently  that  the  map 
does  not  represent  the  condition  of  Mars  at  any  given 
period;  it  is  only  a  sort  of  topographical  register  of  the  ob- 
servations made  of  this  phenomenon  at  different  times.5 

The  observation  of  the  geminations  is  one  of  the  great- 
est difficulty,  and  can  only  be  made  by  an  eye  well  prac- 
tised in  such  work,  added  to  a  telescope  of  accurate  con- 
struction and  of  great  power.  This  explains  why  it  is  that 
it  was  not  seen  before  1882.  In  the  ten  years  that  have 
transpired  since  that  time  it  has  been  seen  and  described 
at  eight  or  ten  observatories.  Nevertheless,  some  still  deny 
that  these  phenomena  are  real,  and  tax  with  illusion  (or 
even  imposture)  those  who  declare  that  they  have  ob- 
served it. 

Their  singular  aspect,  and  their  being  drawn  with  abso- 
lute geometrical  precision,  as  if  they  were  the  work  of 

5  This  map  may  be  found  in  "  La  Planete  Mars,"  by  Flammarion, 
page  440. — TRANSLATOR. 


THE   PLANET  MARS  159 

rule  or  compass,  have  led  some  to  see  in  them  the  work 
of  intelligent  beings,  inhabitants  of  the  planet.  I  am  very 
careful  not  to  combat  this  supposition,  which  includes 
nothing  impossible.  (lo  mi  guardero  bene  dal  combattere 
questa  supposizione,  la  quale  nulla  include  d'  impossibile.) 
But  it  will  be  noticed  that  in  any  case  the  gemination  can 
not  be  a  work  of  permanent  character,  it  being  certain  that 
in  a  given  instance  it  may  change  its  appearance  and  di- 
mensions from  one  season  to  another.  If  we  should  assume 
such  a  work,  a  certain  variability  would  not  be  excluded 
from  it;  for  example,  extensive  agricultural  labour  and  irri- 
gation upon  a  large  scale.  Let  us  add,  further,  that  the 
intervention  of  intelligent  beings  might  explain  the  geo- 
metrical appearance  of  the  gemination,  but  it  is  not  at  all 
necessary  for  such  a  purpose.  The  geometry  of  Nature  is 
manifested  in  many  other  facts  from  which  are  excluded 
the  idea  of  any  artificial  labour  whatever.  The  perfect 
spheroids  of  the  heavenly  bodies  and  the  ring  of  Saturn 
were  not  constructed  in  a  turning  lathe,  and  not  with  com- 
passes has  Iris  described  within  the  clouds  her  beautiful 
and  regular  arch.  And  what  shall  we  say  of  the  infinite 
variety  of  those  exquisite  and  regular  polyhedrons  in  which 
the  world  of  crystals  is  so  rich?  In  the  organic  world,  also, 
is  not  that  geometry  most  wonderful  which  presides  over 
the  distribution  of  the  foliage  upon  certain  plants,  which 
orders  the  nearly  symmetrical,  starlike  figures  of  the  flowers 
of  the  field,  as  well  as  of  the  animals  of  the  sea,  and  which 
produces  in  the  shell  such  an  exquisite  conical  spiral  that 
excels  the  most  beautiful  masterpieces  of  Gothic  archi- 
tecture? In  all  these  objects  the  geometrical  form  is  the 
simple  and  necessary  consequence  of  the  principles  and 
laws  which  govern  the  physical  and  physiological  world. 
That  these  principles  and  these  laws  are  but  an  indication 
of  a  higher  intelligent  Power  we  may  admit,  but  this  has 
nothing  to  do  with  the  present  argument. 

Having  regard,  then,  for  the  principle  that  in  the  ex- 
planation of  natural  phenomena  it  is  universally  agreed  to 
begin  with  the  simplest  suppositions,  the  first  hypotheses 


160  SCHIAPARELLI 

of  the  nature  and  cause  of  the  geminations  have  for  the 
most  part  put  in  operation  only  the  laws  of  inorganic  Na- 
ture. Thus,  the  gemination  is  supposed  to  be  due  either 
to  the  effects  of  light  in  the  atmosphere  of  Mars,  or  to 
optical  illusions  produced  by  vapours  in  various  manners, 
or  to  glacial  phenomena  of  a  perpetual  winter,  to  which  it 
is  known  all  the  planets  will  be  condemned,  or  to  double 
cracks  in  its  surface,  or  to  single  cracks  of  which  the  images 
are  doubled  by  the  effect  of  smoke  issuing  in  long  lines 
and  blown  laterally  by  the  wind.  The  examination  of  these 
ingenious  suppositions  leads  us  to  conclude  that  none  of 
them  seem  to  correspond  entirely  with  the  observed  facts, 
either  in  whole  or  in  part.  Some  of  these  hypotheses 
would  not  have  been  proposed  had  their  authors  been  able 
to  examine  the  geminations  with  their  own  eyes.  Since 
some  of  these  may  ask  me  directly,  "  Can  you  suggest  any- 
thing better?  "  I  must  reply  candidly,  "  No." 

It  would  be  far  more  easy  if  we  were  willing  to  intro- 
duce the  forces  pertaining  to  organic  Nature.  Here  the 
field  of  plausible  supposition  is  immense,  being  capable 
of  making  an  infinite  number  of  combinations  capable  of 
satisfying  the  appearances  even  with  the  smallest  and  sim- 
plest means.  Changes  of  vegetation  over  a  vast  area,  and 
the  production  of  animals,  also  very  small,  but  in  enormous 
multitudes,  may  well  be  rendered  visible  at  such  a  dis- 
tance. An  observer  placed  in  the  moon  would  be  able  to 
see  such  an  appearance  at  the  times  in  which  agricultural 
operations  are  carried  out  upon  one  vast  plain — the  seed- 
time and  the  gathering  of  the  harvest.  In  such  a  manner 
also  would  the  flowers  of  the  plants  of  the  great  steppes  of 
Europe  and  Asia  be  rendered  visible  at  the  distance  of 
Mars — by  a  variety  of  colouring.  A  similar  system  of 
operations  produced  in  that  planet  may  thus  certainly  be 
rendered  visible  to  us.  But  how  difficult  for  the  Lunarians 
and  the  Areans  to  be  able  to  imagine  the  true  causes  of 
such  changes  of  appearance  without  having  first  at  least 
some  superficial  knowledge  of  terrestrial  nature!  So  also 
for  us,  who  know  so  little  of  the  physical  state  of  Mars,  and 


THE   PLANET   MARS  l6l 

nothing  of  its  organic  world,  the  great  liberty  of  possible 
supposition  renders  arbitrary  all  explanations  of  this  sort 
and  constitutes  the  gravest  obstacle  to  the  acquisition  of 
well-founded  notions.  All  that  we  may  hope  is  that  with 
time  the  uncertainty  of  the  problem  will  gradually  diminish, 
demonstrating  if  not  what  the  geminations  are,  at  least 
what  they  can  not  be.  We  may  also  confide  a  little  in  what 
Galileo  called  "  the  courtesy  of  Nature,"  thanks  to  which 
a  ray  of  light  from  an  unexpected  source  will  sometimes 
illuminate  an  investigation  at  first  believed  inaccessible  to 
our  speculations,  and  of  which  we  have  a  beautiful  example 
in  celestial  chemistry.  Let  us  therefore  hope  and  study. 


ii 


METEORITES 
AND  STELLAR  SYSTEMS 

BY 

GEORGE   HOWARD   DARWIN 


METEORITES   AND   STELLAR   SYSTEMS1 

IT  is  only  within  the  last  few  years  that  photographic 
processes  have  been  so  far  perfected  as  to  make  it  pos- 
sible to  photograph  a  faintly  luminous  celestial  object. 
The  success  attained  has  already  been  so  great  that  we  are 
made  aware  of  the  existence  of  a  multitude  of  stars  which 
would  never  have  been  otherwise  perceived,  even  with  the 
finest  telescope  and  under  the  purest  air.  The  sensitized 
plate  sums  up  the  effects  of  light,  so  that  under  prolonged 
exposure  even  a  very  faint  light  at  length  produces  its 
mark.  In  this  respect  the  advantage  is  all  on  the  side  of 
the  photograph  as  compared  with  the  eye,  for  prolonged 
gazing  is  actually  detrimental  to  the  acuteness  of  vision. 

The  exposure  necessary  for  an  ordinary  photograph  in 
the  broad  daylight  may  be  only  a  fraction  of  a  second,  but 
with  the  feeble  light  of  the  stars  three  or  four  hours  are 
found  to  be  necessary  or  advantageous.  Fortunately  for 
the  astronomer  the  heavens  move  uniformly,  and  the  in- 
strument can  be  made  to  follow  the  object  by  clockwork. 
But  as  clocks  are  imperfect,  the  motion  of  the  photographic 
telescope  has  to  be  constantly  regulated  by  hand,  so  as  to 
keep  exact  pace  with  a  star,  which  is  viewed  through  a 
second  telescope  attached  to  the  first  one.  It  may  easily 
be  conceived  that  it  has  required  an  enormous  amount  of 
skill  and  patience  to  attain  to  the  present  high  degree  of 
perfection.  But  the  details  of  celestial  photography  are 
outside  the  scope  of  this  paper,  and  I  am  only  concerned 
with  some  of  the  conclusions  which  have  been  drawn  from 
the  photographic  method. 

1  From  "  The  Century  Magazine,"  October,  1890.    By  special  per- 
mission of  the  Century  Company. 

165 


1 66  DARWIN 

Mr.  Isaac  Roberts,  of  Liverpool,  has  recently  photo- 
graphed a  portion  of  the  heavens,  embracing  about  four 
square  degrees  in  the  constellation  of  Cygnus,  and  he  esti- 
mates that  his  plate  shows  about  sixteen  thousand  stars, 
none  of  which  are,  I  believe,  visible  to  the  naked  eye.  A 
good  idea  may  be  formed  of  this  picture  by  imagining  a 
sheet  of  dark  paper  thoroughly  splashed  with  whitewash. 
The  recent  advance  of  celestial  photography  is  well  illus- 
trated by  the  fact  that  this  same  portion  of  the  heavens, 
when  photographed  in  1885,  appeared  to  contain  only 
about  five  thousand  stars.  Thus  four  years  has  tripled  the 
number. 

Four  square  degrees  comprise  only  about  a  ten-thou- 
sandth of  the  whole  heavens,  and  if  space  were  everywhere 
as  thickly  peopled  as  the  constellation  of  the  Swan,  the 
whole  number  of  stars  photographically  visible  would  reach 
the  stupendous  total  of  one  hundred  and  sixty-seven  mil- 
lions. But  the  Milky  Way  runs  through  Cygnus,  and  this 
is  a  crowded  portion  of  the  heavens.  Yet  there  is  little 
doubt  that  a  hundred  millions  of  stars  would  already  be 
perceptible  if  the  whole  heavens  were  surveyed  with  equal 
thoroughness. 

These  celestial  photographs  bring  vividly  before  us  the 
utter  .insignificance  of  this  world  and  of  ourselves;  for  our 
planet  is  of  almost  contemptible  smallness,  and  our  sun  is 
certainly  a  star  of  no  great  magnitude. 

And  yet  it  is  nearly  twice  as  far  from  the  sun's  center 
to  his  surface  as  from  here  to  the  moon,  and  the  planet 
Neptune  is  distant  nearly  three  thousand  million  miles  from 
the  sun.  The  mind  fails  to  grasp  such  a  number  of  stars 
as  a  hundred  million,  and  a  limit  to  the  perfection  of  celes- 
tial photography  has  certainly  not  yet  been  reached. 

Each  of  these  millions  of  stars  has  its  history,  and  there 
are  among  them  representatives  of  every  stage  of  evolution. 
If  they  were  not,  even  with  the  telescope,  mere  specks  of 
light,  we  might  see  the  whole  process  before  us,  and  might 
study  them  like  the  objects  in  a  museum. 

Among  the  stars  there  are,  however,  small  luminous 


METEORITES   AND  STELLAR  SYSTEMS  167 

clouds  called  nebulae,  which  are  not  immeasurably  small. 
They  have,  of  course,  been  examined  with  all  the  finest 
telescopes  for  many  years,  and  many  strange  vagaries  in 
their  structure  have  been  noted. 

It  is  true  that  we  know  the  stars  and  nebulae  to  be  made 
of  materials  found  on  the  earth,  and  we  can  estimate  ap- 
proximately how  hot  they  are,  and  which  are  old  in  their 
history  and  which  are  young.  All  this  has  been  discovered 
by  means  of  that  wonderful  instrument,  the  spectroscope, 
but  it  can  not  show  us  their  shapes  and  structures.  Within 
the  last  few  months,  however,  there  is  reason  to  hope  that 
the  telescopic  photograph  may  really  bring  before  us  in  an 
intelligible  shape  many  objects  from  the  celestial  museum. 

Notwithstanding  the  paucity  of  definite  knowledge, 
many  theories  have  been  propounded  as  to  the  sequence 
of  changes  through  which  the  solar  system  has  passed.  The 
most  celebrated  of  these  is  that  associated  with  the  names 
of  the  great  mathematician  Laplace  and  of  the  philosopher 
Kant.  It  is  remarkable  that  substantially  the  same  theory 
should  have  been  independently  formulated  by  two  men 
whose  intellects  were  so  different. 

They  both  suggested  that  the  matter  which  now  forms 
the  sun  and  the  planets  existed  in  primitive  times  as  a 
globular  nebula  of  highly  rarefied  gas  in  slow  rotation,  and 
their  theory  is  accordingly  generally  known  as  the  nebular 
hypothesis. 

Every  portion  of  this  nebula  of  course  attracted  every 
other  portion,  and  therefore  there  must  have  been  a  con- 
densation at  the  centre,  at  which  point  a  dense  nucleus 
must  ultimately  have  formed. 

The  rotation  made  the  nebula  fly  out  like  a  trundled 
mop,  but  the  outward  tendency  was  counteracted  by  attrac- 
tion. This  battle  between  the  attraction  due  to  gravitation 
and  the  repulsion  due  to  rotation  caused  a  flattening  of  the 
globe,  so  that  it  became  orange-shaped. 

The  gas  of  which  the  nebula  was  composed  possessed 
heat;  the  central  part  being  probably  very  hot  and  the 
external  part  very  cold,  as  estimated  by  terrestrial  stand- 


DARWIN 

ards.  As  the  energy  of  heat  was  gradually  lost  by  radiation 
into  space  the  globe  shrank,  and  at  the  same  time  the  cen- 
tral portion  became  still  hotter. 

In  consequence  of  the  shrinkage,  the  rate  of  rotation 
was  increased.  This  mechanical  effect  may  easily  be  illus- 
trated thus:  If  I  whirl  a  stone  attached  to  a  string  and  let 
the  string  wind  itself  up  on  my  finger,  the  stone  will  whirl 
faster  and  faster  as  the  string  shortens. 

Lastly,  with  increased  rate  of  rotation  the  increased  re- 
pulsion due  to  centrifugal  force  augmented  the  flattening 
of  the  globe.  At  length  a  time  arrived  when  the  globe  was 
flattened  until  it  became  more  like  a  disk  than  a  globe,  and 
gravitation  was  then  no  longer  capable  of  holding  it  to- 
gether in  a  single  shape. 

Everywhere  in  the  nebula  the  gas  was  being  pressed  by 
the  surrounding  gas,  attracted  toward  the  centre  of  the 
nebula,  and  repelled  by  centrifugal  force  away  from  the  axis 
of  rotation.  The  attraction  diminishes  and  the  repulsion 
increases  the  farther  we  go  from  the  centre.  If  at  a  place 
near  the  edge  of  the  disklike  globe  the  attraction  and 
repulsion  are  just  equal  to  one  another,  pressure  is  not 
called  into  play  in  keeping  the  gas  in  its  place.  At  this 
distance  from  the  centre,  then,  the  gas  which  is  outside 
does  not  press  at  all  on  that  which  is  inside,  and  the 
inner  gas  may  part  company  with  the  outer  gas  without 
disturbance  to  it. 

In  fact,  according  to  the  nebular  hypothesis,  when  the 
flattening  had  reached  a  certain  degree  a  ring  separated 
itself  from  the  equatorial  regions.  The  central  portion, 
thus  relieved,  regained  a  more  globular  shape,  continued  to 
contract  and  to  spin  quicker,  until  a  second  crisis  super- 
vened, when  another  ring  was  shed.  A  succession  of  rings 
was  thus  formed,  and  after  the  detachment  of  the  last  the 
central  portion,  continuing  to  contract,  at  length  formed 
the  sun. 

Each  ring,  as  soon  as  it  was  free,  began  to  aggregate 
round  some  denser  portion  in  its  periphery.  Subordinate 
nebulae  were  thus  formed,  and  they  in  their  turn  contracted 


METEORITES  AND  STELLAR  SYSTEMS     169 

and  shed  rings.  The  nucleus  of  the  secondary  nebulae 
formed  the  planets,  and  their  rings  condensed  into  satellites. 

This  is  an  outline  of  the  celebrated  nebular  hypothesis. 
I  shall  now  show  what  an  interesting  confirmation  this 
theory  receives  from  a  recent  photograph. 

There  is  in  the  constellation  of  Andromeda  a  nebula  so 
remarkable  that  its  nebulous  character  was  recognised  even 
long  before  the  invention  of  the  telescope. 

This  nebula  was  first  photographed  with  conspicuous 
success,  in  October,  1888,  by  Mr.  Roberts,  and  again  on 
the  29th  of  the  following  December,  1888. 

The  result  is  of  the  greatest  interest,  for  in  it  we  actually 
see  what  Laplace  pictured  with  his  mind's  eye.  There  is 
a  bright  central  condensation  surrounded  by  ring  after  ring, 
gradually  dying  away  into  faintness. 

In  one  of  the  rings  there  is  a  region  of  greater  bright- 
ness, which  may  fairly  be  interpreted  as  the  centre  of  aggre- 
gation for  a  planet.  At  another  place  which  is  clearly  more 
remote  from  the  centre,  although  brought  nearer  by  fore- 
shortening, we  have  a  brilliant  round  luminous  ball — surely 
a  planetary  nebula  already  formed.  At  a  much  greater  dis- 
tance there  is  an  elongated  nebulosity,  which  we  may  con- 
jecture to  be  a  planetary  nebula  seen  edgewise,  but  in  a 
further  state  of  advance  than  the  other.  It  is  worthy  of 
notice  that  the  remote  planets  Neptune  and  Uranus  rotate 
about  axes  nearly  in  the  plane  of  their  orbits,  and  from  the 
direction  of  elongation  of  this  subordinate  nebula  it  seems 
as  though  the  like  must  be  true  here. 

In  1848  Bond  measured  the  positions  of  these  two 
bright  small  nebulae  relatively  to  the  large  one,  and  they 
seem  to  have  changed  their  positions  since  that  date.  This 
confirms  the  theory  that  they  are  planets,  but  it  must  be 
admitted  that  measurements  with  reference  to  an  ill-defined 
object  like  a  nebula  are  hard  to  make  with  precision. 

I  should  suppose  this  to  be  the  greatest  triumph  yet 
achieved  by  celestial  photography,  and  I  owe  my  sincere 
thanks  to  Mr.  Roberts  for  allowing  me  to  reproduce  it. 

But  these  pictures,  while  confirming  the  substantial 


DARWIN 

truth  of  the  nebular  hypothesis,  fail  to  clear  up  many  of 
the  obscurities  which  surround  the  evolution  of  a  planetary 
system.  There  is  one  difficulty  indeed  so  tundamental  that 
it  has  led  some  astronomers  virtually  to  throw  over  the 
whole  theory,  and  it  forms  the  special  object  of  this  essay 
to  discuss  it. 

It  is  the  very  essence  of  the  nebular  hypothesis  that  the 
nebula  should  be  formed  of  continuous  gas,  one  part  of 
which  exercises  a  pressure  on  another  part;  for  we  have 
seen  how  gaseous  pressure  is  instrumental  in  imparting  the 
globular  form  to  the  whole,  and  how  when  the  globe  loses 
heat  and  shrinks  it  is  just  along  that  line  where  the  pres- 
sure vanishes  that  the  ring  splits  off. 

Now,  there  is  no  perceptible  trace  in  the  solar  system 
of  that  all-pervading  gas  from  which  the  whole  is  supposed 
to  have  been  evolved;  for  the  planets  do  not  suffer  any 
sensible  retardation  in  their  motion  round  the  sun,  as  would 
be  the  case  if  they  were  moving  through  even  a  highly 
rarefied  gas. 

On  the  other  hand,  there  is  evidence  of  abundance  of 
solid  bodies  flying  through  space.  When  these  bodies  meet 
our  atmosphere  they  glow  up  white-hot  with  friction,  and 
are  called  falling  stars  or  meteorites.  Though  they  are  gen- 
erally dissipated  into  dust  in  their  passage  through  the  air, 
yet  once  in  a  while  one  of  them  owes  its  preservation  to  its 
greater  size,  and  falls  on  the  earth.  We  thus  know  them  to 
be  strange-looking  stones,  largely  composed  of  iron. 

The  ring  which  surrounds  the  planet  Saturn  was  obvi- 
ously suggestive  of  the  nebular  hypothesis  to  the  minds  of 
Laplace  and  Kant.  But  it  has  been  conclusively  proved 
by  the  researches  of  Roche  and  Clerk  Maxwell  to  consist 
of  a  swarm  of  loose  stones — a  shower  of  brickbats,  as  Max- 
well was  fond  of  calling  it.  And  now  within  the  last  three 
years  spectroscopic  research  has  led  Mr.  Lockyer  to  sug- 
gest that  the  luminous  gas,  which  undoubtedly  forms  the 
visible  portion  of  the  nebulae,  is  simply  gas  volatilized  from 
the  solid  state  and  rendered  incandescent  by  the  violent 
impact  of  meteoric  stones. 


METEORITES  AND  STELLAR  SYSTEMS  171 

These  gases,  he  tells  us,  cool  quickly,  cease  to  be  lumi- 
nous, and  condense  again  into  the  solid  state,  but  the  colli- 
sions being  incessant,  the  whole  nebula  shines  with  a  steady 
light.  Mr.  Lockyer  supports  his  view  by  an  elaborate 
comparison  of  the  spectra  of  stars  and  nebulae  with  those 
of  actual  meteorites,  fused  by  the  electric  spark  in  the 
laboratory.  I  have  not  the  knowledge  of  spectroscopy 
which  would  be  necessary  to  examine  his  theory,  but  his 
general  conclusions  seem  to  be  of  the  highest  importance 
in  the  study  of  stellar  systems. 

All  these  lines  of  observation  conspire  to  indicate  that 
the  immediate  antecedent  of  the  sun  and  planets  was  not  a 
continuous  gas,  but  a  swarm  of  loose  stones.  And  yet  the 
nebular  hypothesis  seems  as  good  as  proved  by  this  photo- 
graph of  Mr.  Roberts.  Here,  then,  we  find  ourselves  in 
a  dilemma;  on  the  one  hand  we  have  the  meteoric  theory 
denying  the  continuity  of  the  matter  which  forms  the 
nebulae,  while  on  the  other  hand  the  nebular  hypothesis 
demands  such  continuity.  I  wish  to  emphasize  this  point: 
either  a  nebula  is  made  of  a  cooling  gas,  such  as  hydrogen, 
nitrogen,  oxygen,  and  the  vapours  of  metals,  or  it  is  not 
so.  The  nebular  hypothesis  apparently  says  it  must  con- 
sist of  gas,  while  this  is  denied  by  the  strong  evidence  that 
it  consisted  of  an  enormous  number  of  stones.  It  seems  at 
first  that  either  the  nebular  hypothesis  or  the  meteoric 
theory  must  be  untrue. 

I  believe,  nevertheless,  that  there  is  a  way  in  which 
these  conflicting  ideas  may  be  brought  into  harmony  and 
made  to  re-enforce  one  another,  and  the  special  object  of 
this  paper  is  to  effect  such  a  reconciliation.  But  before 
coming  to  that  we  must  leave  for  a  time  the  world  of  stars, 
and  must  consider  the  ultimate  structure  of  a  gas,  such 
as  the  air  we  breathe.  A  gas  is  now  known  to  consist  of 
ultra-microscopic  molecules,  all  exactly  alike  in  weight, 
shape,  and  structure.  Although  they  are  invisible,  they 
can  be  counted  and  timed;  there  are  found  to  be  millions 
in  a  cubic  inch  of  air,  moving  indiscriminately  in  all  direc- 
tions, with  great  velocity.  For  example,  in  the  air,  at  a 


1 72  DARWIN 

temperature  of  60°  Fahr.,  their  average  speed  is  1,570  feet 
a  second — half  as  fast  again  as  the  velocity  of  sound.  The 
temperature  of  a  gas  simply  depends  on  the  rate  at  which 
the  molecules  are  moving.  Millions  of  times  in  each  sec- 
ond each  one  of  these  molecules  happens  by  chance  to 
strike  one  of  its  neighbours,  and  the  two  which  have  struck 
rebound  from  each  other  as  though  they  were  of  India 
rubber,  or  at  any  rate  after  such  an  encounter  they  behave 
as  though  they  were  perfectly  elastic.  If  we  could  watch 
the  crowd  we  should  see  the  individuals  darting  about  in 
a  zigzag  course,  being  deflected  into  a  new  direction  at 
each  collision.  I  have  often  been  reminded  of  this  so-called 
kinetic  theory  of  gases  when  watching  the  dance  of  a  little 
swarm  of  house-flies  as  they  zigzag  about  and  sharply 
change  their  paths,  when,  for  a  second,  two  of  them  get 
entangled  together.  Perhaps  this  familiar  example  may 
help  the  reader  to  realize  the  dance  of  the  molecules  in 
a  gas. 

The  incessant  agitation  of  molecules  is  quite  independ- 
ent of  winds  and  draughts,  and  when  as  many  molecules  are 
going  in  any  one  direction  as  in  any  other  we  consider 
the  air  to  be  calm.  What  we  call  a  wind  is  when  more 
molecules  are  going  in  the  direction  of  the  wind  than  in 
any  other. 

The  molecules  of  a  gas  are  not  aimed  at  one  another, 
and  as  a  collision  is  all  a  matter  of  chance,  it  is  clear  that 
a  molecule  is  sometimes  nearly  stopped,  sometimes  im- 
pelled faster,  and  sometimes  merely  deflected.  Thus  they 
are  moving  with  all  possible  speeds,  but  the  great  majority 
are  moving  with  about  the  average  speed  of  the  whole 
crowd. 

According  to  this  theory,  the  pressure  of  a  gas  is  merely 
the  cannonade  of  millions  of  molecules  against  the  side  of 
the  vessel  containing  the  gas;  as  the  number  of  impacts 
per  square  inch  and  per  second  is  enormous,  the  effect  is 
indistinguishable  from  that  of  continuous  pressure. 

We  are  accustomed  to  make  statistical  inquiries  into 
any  question  affecting  groups  of  men,  and  the  same  method 


METEORITES  AND  STELLAR  SYSTEMS  173 

has  to  be  applied  to  the  collisions  of  the  molecules  of  a 
gas.  These  very  complex  statistics  have  been  profoundly 
studied  by  Maxwell,  Clausius,  and  others,  and  they  have 
shown  how  to  compute  from  the  temperature  and  density 
of  a  gas  the  average  velocity  of  its  molecules,  the  average 
frequency  of  collision,  and  the  average  distance  travelled 
between  successive  collisions. 

It  will  not  be  possible  to  go  into  these  difficult  questions 
at  present,  and  it  must  be  accepted  that  a  gas  is  actually 
composed  of  constantly  colliding  particles  or  molecules. 
But  when  we  look  at  a  gas  from  this  point  of  view  we 
must  take  care  not  to  confuse  the  single  molecule  with  the 
gas  of  which  it  forms  part.  Gas  is  merely  our  word  for 
a  crowd  of  molecules,  much  as  nation  is  a  word  for  a  crowd 
of  men.  National  history  is  no  more  to  be  learned  from 
the  doings  and  character  of  a  single  man  than  the  prop- 
erties of  a  gas  are  to  be  learned  from  the  doings  and  char- 
acter of  a  single  molecule.  In  "  gas  "  and  in  "  nation  " 
the  relationship  of  all  to  all  is  involved. 

Now  that  the  internal  structure  of  common  air  has  been 
explained,  let  us  examine  a  little  more  closely  its  relation  to 
ourselves. 

If  we  were  to  shrink  to  a  ten-millionth  of  our  actual 
size,  how  different  would  air  seem!  It  would  then  seem  to 
consist  of  cannon-balls  flying  about  in  all  directions,  at  rare 
intervals,  and  at  a  prodigious  rate.  And  yet  the  supposed 
change  in  us  would  not  have  affected  the  nature  of  air,  and 
it  would  still  be  a  continuous  gas  to  our  former  senses. 
Thus  the  description  we  should  give  of  a  gas  is  all  a  mat- 
ter of  the  relative  scales  of  largeness  of  ourselves  and  of 
the  gas. 

Now,  this  theory  of  a  gas  affords  the  idea  by  which  I 
seek  to  reconcile  the  conflicting  theories  of  the  evolution 
of  stellar  systems.  My  suggestion  is  that  celestial  nebulse 
are  drawn  on  so  large  a  scale  that  meteorites  may  be  treated 
as  molecules,  and  that  the  collisions  of  meteorites  are  so 
frequent  that  the  whole  swarm  will  behave  as  though  it 
were  a  gas.  The  relationship  of  us  men  to  this  coarse- 


174 


DARWIN 


grained  meteoric  medium  is  exactly  that  of  the  ideal  pyg- 
mies to  common  air. 

But  it  is  not  enough  to  make  such  a  suggestion  as  this; 
the  details  of  the  idea  must  be  examined.  We  must  con- 
sider what  may  be  supposed  to  happen  when  two  stones 
clash  together,  and  must  see  whether  they  can  come  into 
collision  often  enough  to  make  the  swarm  into  a  kind 
of  gas. 

In  comparing  the  behaviour  of  meteorites  to  the  mole- 
cules of  a  gas  it  will  naturally  occur  to  inquire  whether  they 
can  be  supposed  to  possess  that  high  degree  of  elasticity 
which  is  necessary  for  a  kinetic  theory.  I  believe  that  this 
question  may  be  answered  in  the  affirmative.  Meteoric 
stones  move  with  speeds  which  are  very  great  according  to 
our  terrestrial  notions;  and  even  without  Mr.  Lockyer's 
direct  spectroscopic  evidence  we  could  not  doubt  that 
enough  heat  is  generated  in  a  collision  to  volatilize  part  of 
the  solid  matter  of  each  stone,  and  to  make  the  gas  incan- 
descent. Now,  a  sudden  generation  of  gas  at  the  point  of 
contact  of  the  two  stones  would  be  exactly  like  the  explo- 
sion of  a  charge  of  gunpowder  between  them,  and  they 
would  be  blown  apart  with  great  violence.  As  far  as  re- 
gards their  velocities  after  collision  the  result  would  be 
much  the  same  as  though  they  were  highly  elastic,  although 
this  virtual  elasticity  is  of  quite  a  different  character  from 
that  tendency  of  a  strained  solid  to  recover  its  shape,  which 
constitutes  ordinary  elasticity.  I  may  call  to  mind,  as  an 
example  of  an  abnormal  elasticity  of  somewhat  the  same 
sort,  how  a  leaden  bullet  bounds  from  the  surface  of  the 
sea,  although  lead  is  a  very  inelastic  solid,  and  water  is  not 
solid  at  all. 

It  is  not  claimed  that  these  considerations  prove  abso- 
lutely that  two  meteorites  would  bound  from  each  other  as 
if  they  were  very  elastic,  but  it  seems  highly  probable  that 
they  would  do  so,  and  the  matter  is  not  susceptible  of  strict 
proof.  But  granting  the  elasticity,  there  is  another  point 
to  consider. 
>  If  two  stones  meet,  the  chance  of  their  fracture  is 


METEORITES  AND   STELLAR  SYSTEMS  175 

greater  if  they  are  great  than  if  they  are  small,  and  the 
breakage  may  go  on  only  until  a  certain  size,  dependent  on 
the  average  velocity  of  the  meteorites,  is  reached,  after 
which  it  may  become  unimportant. 

When  the  gases  generated  on  collision  cool  they  will 
condense  into  a  metallic  rain,  and  this  may  fuse  with  old 
meteorites.  Some  actual  meteorites  show  signs  of  the 
fusion  of  many  distinct  nuclei.  Thus  there  are  both  ab- 
stract reason  and  direct  evidence  in  support  of  occasional 
fusions.  The  mean  size  of  meteorites  in  a  swarm  probably 
depends  on  the  balance  between  the  opposing  forces  of 
breakage  and  fusion. 

No  doubt  when  two  stones  meet  directly  each  is  shat- 
tered to  fragments.  But  glancing  collisions  must  be  indefi- 
nitely more  frequent,  and  in  these  we  may  suppose  that 
fracture  is  comparatively  rare,  and  virtual  elasticity  great. 

The  possible  frequency  of  fracture  undoubtedly  does 
present  a  difficulty  in  the  theory,  for  it  would  seem  as 
though  the  whole  swarm  of  stones  must  gradually  degrade 
into  dust.  There  must  be  some  way  out  of  this  difficulty, 
for  meteorites  of  considerable  size  fall  upon  the  earth,  and 
unless  Mr.  Lockyer  has  misinterpreted  the  spectroscopic 
evidence,  the  nebulae  do  now  consist  of  meteorites. 

I  hold  that  these  considerations  justify  us  in  maintain- 
ing a  rough  similarity  between  meteoric  stones  and  the 
molecules  of  a  gas,  as  far  as  regards  the  actual  collisions. 
If  this  is  so,  what  is  called  the  temperature  of  a  gas  must 
be  translated  as  meaning  the  average  energy  of  motion  of 
the  meteorites. 

We  must  now  go  on  to  consider  how  often  meteorites 
collide,  and  try  to  discover  whether  a  swarm  of  meteorites 
possesses  a  fine  enough  texture  to  permit  the  applicability 
of  the  theory.  For  this  part  of  the  discussion  numerical 
calculations  are  necessary.  Calculations  require  numerical 
data,  and  these  can  only  be  derived  from  a  known  system, 
and  of  course  the  only  one  known  with  any  precision  is  the 
solar  system.  The  fineness  of  grain  is  obviously  independ- 
ent of  the  amount  of  flattening  of  the  nebula  which  arises 


DARWIN 

from  rotation.  In  order,  therefore,  to  simplify  the  matter, 
and  to  consider  one  thing  at  a  time,  the  nebula  is  supposed 
to  be  one  in  which  there  is  no  rotation. 

The  weight  of  the  sun  in  pounds  is  four  with  thirty 
zeros  after  it,  and  I  suppose  the  sun  to  be  broken  up  into 
that  number  of  meteorites,  each  weighing  one  pound.  If 
the  meteorites  are  supposed  to  be  of  iron  their  exact  size 
is  known,  because  the  dimensions  of  a  pound  of  iron  are 
known.  This  supposition  as  to  the  weight  and  size  of  the 
meteorites  is  merely  adopted  as  a  type,  but  it  suffices  for 
our  present  purpose.  These  one-pound  iron  stones  are  to 
be  distributed  in  a  swarm  extending  beyond  the  present 
orbit  of  Neptune.  To  give  numerical  precision,  let  us 
suppose  that  the  swarm  extends  half  as  far  again  as  Nep- 
tune's orbit — that  is  to  say,  let  it  extend  to  forty-five  times 
the  distance  of  the  earth  from  the  sun. 

In  this  condition  the  nebula  is  of  extreme  tenuity,  and 
if  the  stones  are  not  then  too  sparse  to  make  the  swarm 
behave  like  a  gas,  it  will,  a  fortiori,  behave  like  a  gas  when 
the  nebula  has  shrunk  and  the  stones  are  more  closely 
packed.  The  supposition  made  as  to  the  extension  of 
the  solar  swarm,  therefore,  puts  the  theory  to  a  severe 
test. 

In  the  case  of  a  town,  density  of  population  means  the 
number  of  people  to  the  square  mile,  and  for  meteorites 
what  we  may  still  call  the  density  of  population  is  the 
number  to  the  cubic  mile.  The  swarm  is  not  supposed  to 
be  rotating,  and  is  therefore  a  perfect  globe,  and  the  layers 
of  equal  density  of  population  are  also  spheres. 

The  stones  will  not  be  evenly  distributed  in  space,  but 
the  density  of  population  will  be  much  greater  toward  the 
middle  than  toward  the  outside.  The  reason  of  this  is  that 
every  stone  is  attracted  toward  the  middle,  and  is  only 
prevented  from  yielding  to  the  tendency  by  the  blows  it 
receives  from  its  neighbours.  Think  of  a  crowd  struggling 
for  tickets  at  a  railway  station,  and  you  have  a  picture  of 
what  happens.  The  men  squeeze  and  push  and  sway  about, 
but  the  crowd  remains  of  about  the  same  density  at  each 


METEORITES  AND  STELLAR   SYSTEMS  177 

distance  from  its  middle.  So  in  the  swarm  the  dance  of  the 
meteorites  is  incessant,  but  it  arranges  itself  automatically 
into  a  steady  condition,  in  which  the  density  of  population 
has  no  tendency  to  shift. 

It  is  natural  to  ask  why  the  stones  should  be  moving 
at  all,  and  how  they  acquired  their  great  speed.  This  is 
a  question  that  imperatively  demands  an  answer,  and  we 
are  able  to  answer  it  with  certainty.  They  derive  their 
speed  from  gravitation;  they  have  fallen  in  from  a  great 
distance  toward  a  centre  of  aggregation.  A  description  of 
the  way  in  which  they  may  have  come  together  will  make 
it  clear  why  they  are  moving,  and  will  also  give  the  reader 
some  idea  of  how  the  actual  velocity  may  be  calculated  in  a 
swarm  of  given  mass  and  size. 

Imagine  that  somewhere  in  space  there  is  an  aggregation 
of  meteorites — no  matter  how  it  got  there — and  conceive 
a  stone  released  from  a  state  of  rest  at  a  very  great  distance. 
Under  the  attraction  of  gravitation  the  stone  falls  toward 
the  centre  of  aggregation,  and  on  reaching  the  confines  of 
the  swarm  it  will  have  acquired  a  certain  velocity.  It  then 
penetrates  the  swarm  for  some  uncertain  distance,  until  it 
happens  to  strike  another  meteorite.  Henceforth  its  path 
is  zigzag,  as  it  happens  to  strike,  and  we  need  not  suppose 
ourselves  to  watch  it  any  longer,  since  it  has  become  one  of 
the  swarm.  It  is,  however,  important  to  remark  that  the 
supposed  visitant  from  outside  space  has  imported  energy 
of  motion  into  the  system,  which  energy  it  gradually  com- 
municates to  its  neighbours  by  collisions;  it  has  also  in- 
creased the  mass  of  the  swarm.  When  another  stone  is 
allowed  to  fall  in,  since  it  is  attracted  by  a  slightly  greater 
mass,  it  arrives  at  the  swarm  with  slightly  greater  speed 
than  the  first.  So  if  we  imagine  the  swarm  to  be  increased 
by  the  addition  of  stone  after  stone,  we  see  that  in  the 
course  of  accretion  the  energy  of  agitation  of  its  constitu- 
ent meteorites  gradually  increases.  Also  the  volume  of  the 
globe  throughout  which  (if  anywhere)  the  swarm  possesses 
the  mechanical  properties  of  a  gas  is  at  the  same  time 
gradually  increased.  We  must  suppose  that  at  length  all 

12 


178  DARWIN 

the  stones  in  that  part  of  space  are  exhausted,  the  materials 
of  the  nebula  are  collected,  and  it  only  remains  for  them 
to  work  out  their  future  fate. 

By  this  sort  of  reasoning  we  find  out  how  fast  the  stones 
are  moving,  but  it  is  proper  to  add  that  an  important  cor- 
rection has  to  be  applied  to  allow  for  the  fact  that  at  each 
collision  some  speed  is  lost.  In  the  process  of  settling  into 
the  steady  condition  each  stone  retains  only  seven  tenths  of 
the  velocity  it  would  have  had  if  it  were  a  fresh  arrival  from 
space. 

It  will  make  no  material  difference  in  these  results  by 
what  process  the  stones  were  brought  together,  and  this 
account  which  I  have  just  given  of  the  formation  of  a 
swarm  is  not  intended  as  a  contribution  to  its  history,  but 
is  only  meant  to  render  intelligible  the  mechanical  prin- 
ciples involved,  and  to  show  in  a  general  way  how  the 
matter  may  be  subjected  to  calculation. 

By  such  a  line  of  argument  as  this  I  found  that,  when 
the  solar  swarm  extended  half  as  far  again  as  the  planet 
Neptune,  in  the  central  region  the  stones  were  moving  at 
an  average  rate  of  three  miles  a  second — two  hundred  times 
as  fast  as  a  fast  train — but  that  in  the  outer  portion  of  the 
swarm  the  velocity  was  less. 

We  have  now  to  find  out  how  often  the  stones  came 
into  collision,  how  far  they  travelled  beween  collisions,  and 
whether  the  collisions  were  frequent  enough  to  allow  us  to 
consider  the  whole  nebula  as  a  kind  of  gas,  as  is  demanded 
by  the  nebular  hypothesis. 

To  how  small  a  pygmy  would  air  still  be  air?  The  an- 
swer is  that  the  pygmy  must  be  just  large  enough  to  be 
struck  so  often  that  he  loses  the  sensation  of  the  individual 
blows,  and  is  only  aware  of  their  average  effect.  It  will 
insure  this  if  he  is  struck  hundreds  or  thousands  of  times 
in  the  average  interval  between  two  collisions  of  a  molecule 
of  air;  or  we  may  say  that  his  bulk  must  be  great  enough 
to  contain  thousands  of  molecules,  or  his  length  thousands 
of  times  as  great  as  the  average  path  traversed  by  a  mole- 
cule between  two  successive  collisions.  These  conditions 


METEORITES   AND   STELLAR  SYSTEMS  179 

are  amply  satisfied  in  the  relationship  of  the  smallest  micro- 
scopic animalcule  to  our  air. 

It  must,  however,  be  a  giant  who  would  not  feel  the 
individual  blows  of  meteorites,  but  only  realize  their  aver- 
age effects.  If  we  might  consider  the  nebula,  as  a  whole, 
to  be  a  living  being,  we  might  say  that  if  she  is  to  behave 
like  a  gas  she  must  realize  herself  as  a  gas,  and  so  she  must 
be  the  giant  to  whose  perceptions  the  meteoric  nebula  is 
to  be  a  gas;  hence  the  giant  must  not  be  larger  than  the 
nebula  itself. 

It  would  not  be  easy  to  explain  the  exact  reasoning  by 
which  a  comparison  is  made  between  the  dimensions  of  the 
giant  and  the  texture  of  the  nebula  at  every  part  of  itself. 
It  must  suffice  to  say  that  the  comparison  is  best  clothed  in 
a  form  which  may  appear  something  quite  different,  but 
which  is  really  substantially  the  same. 

Except  at  the  moment  of  a  collision,  a  meteorite  is  like 
a  very  small  planet,  and  accordingly  moves  in  a  curved  orbit, 
but  at  each  collision  it  starts  in  a  new  orbit. 

I  say,  then,  that  the  nebula  will  behave  sufficiently  like 
a  gas  to  allow  the  nebular  hypothesis  to  be  true,  if  the 
average  path  of  a  meteorite  between  two  collisions  is  so 
short  that  the  bit  of  orbit  described  departs  very  little  from 
a  straight  line. 

We  now  at  length  come  to  the  numerical  values  to 
which  we  have  been  tending,  and  shall  see  how  often  the 
stones  of  the  solar  nebula  came  into  collision  with  one  an- 
other when  the  nebula  extended  in  a  swarm  of  one-pound 
iron  meteorites  half  as  far  again  from  its  centre  as  the 
present  distance  of  Neptune  from  the  sun. 

In  this  case  I  find  that  at  the  middle  of  the  swarm  a 
meteorite  would  on  the  average  come  into  collision  every 
thirteen  hours,  and  would  travel  140,000  miles  between 
collisions;  at  the  distance  of  the  small  planets,  called  the 
asteroids,  it  would  collide  every  seventeen  hours  and  would 
travel  190,000  miles  between;  at  the  distance  of  Uranus 
the  collisions  would  be  at  intervals  of  twenty-five  days,  and 
the  path  6,000,000  miles;  and  lastly,  at  the  distance  of  Nep- 


l8o  DARWIN 

tune,  the  interval  would  be  one  hundred  and  ninety  days 
and  the  path  28,000,000  miles. 

I  have  said  that  the  criterion  we  have  to  apply  depends 
on  the  amount  of  curvature  of  the  average  path  of  a  stone 
between  two  successive  collisions.  Now,  it  may  be  shown 
that  the  amount  of  departure  from  straightness  is  greater 
the  farther  we  go  from  the  middle  of  the  swarm;  and  I 
find  that  even  at  the  distance  of  Neptune  the  collisions  are, 
speaking  relatively,  so  frequent  that  gravity  only  suffices 
to  draw  the  meteorite  aside  from  the  straight  path  by  one 
sixty-sixth  of  the  path  it  has  traversed.  The  fraction  one 
sixty-sixth  is  then  the  value  of  the  criterion  which  was  to 
be  applied.  Now  one  sixty-sixth  is  so  small  a  fraction  that 
it  may  be  concluded  that  the  meteoric  swarm  passes  the 
proposed  test,  notwithstanding  that  the  great  extension 
which  has  been  attributed  to  the  nebula  strains  the  hy- 
pothesis severely. 

It  follows,  therefore,  that  if  meteorites  possess  virtual 
elasticity,  and  if  breakages  are  counterbalanced  by  fusions, 
then  a  swarm  of  meteorites  provides  a  gaslike  medium  of 
a  fine  enough  structure  to  satisfy  the  demands  of  the  nebu- 
lar hypothesis. 

Some  such  numerical  examination  as  the  foregoing  is 
necessary  in  order  to  assure  us  that  the  quality  of  a  gas 
can  have  been  imparted  to  a  nebula  in  the  suggested  way, 
and  so  to  lift  the  hypothesis  out  of  the  realms  of  mere 
conjecture. 

We  may  conclude  from  this  discussion  that  it  is  possible 
to  justify  the  contention  that  the  meteoric  theory  is  recon- 
cilable with  the  nebular  hypothesis,  and  that  we  may  ac- 
cordingly hold  the  truth  of  both  of  them  at  the  same  time. 

If  space  permitted  I  might  go  on  to  consider  some  of 
the  conclusions  fairly  deducible  from  this  view  of  a  nebula, 
but  it  must  suffice  to  say  that  this  theory  seems  likely  to 
prove  fruitful  in  the  further  elucidation  of  this  complex  and 
necessarily  speculative  subject. 

Up  to  this  time  we  have  been  occupied  with  proving, 
or  rendering  probable,  a  modification  in  Laplace's  theory. 


METEORITES  AND  STELLAR  SYSTEMS  181 

But  it  would  hardly  be  satisfactory  to  leave  the  matter  at 
this  point.  I  wish  it  were  possible  to  gain  an  insight  into 
the  origin  and  previous  history  of  these  stones,  but  on 
these  mysteries  I  have  no  suggestion  to  make.  It  is,  how- 
ever, possible  to  see  pretty  clearly  what  happened  after  the 
nebular  stage,  and  how  all  this  bears  on  the  state  of  things 
of  which  we  are  witnesses  to-day. 

At  the  various  centres  of  condensation  which  we  now 
call  sun,  planets,  and  satellites  the  swarm  of  meteorites  be- 
came denser  and  denser.  The  collisions  were  too  frequent 
to  let  the  gases  cool  and  condense  again,,  and  thus  by  de- 
grees the  meteorites  were  entirely  volatilized.  Thus  round 
these  centres  we  should  have  at  length  a  mass  of  glowing 
gas,  and  toward  the  middle  fluids  and  solids.  All  this  must 
have  occurred  comparatively  early  in  the  case  of  the  sun, 
later  at  the  planets,  and  last  at  the  satellites. 

Outside  of  these  condensations  there  were  numbers  of 
free  meteorites,  but  the  majority  of  the  stones  which  formed 
the  swarm  in  primitive  times  were  already  absorbed,  and 
the  absorption  still  went  on  gradually. 

The  collisions  among  the  free  meteorites  became  rarer, 
because  they  were  scattered  more  sparsely;  and  less  violent, 
because  at  each  successive  collision  some  relative  motion 
was  lost.  Finally,  the  collisions  were  nearly  annulled.  The 
residue  of  the  meteoric  swarm  then  consisted  of  sparse 
flights  of  meteorites,  moving  in  streams.  Such  streams 
give  us  no  evidence  of  their  existence,  except  under  special 
circumstances. 

The  zodiacal  light  is  a  lens-shaped  luminosity,  seen  in 
the  east  or  west  shortly  before  sunrise  or  after  sunset — not 
commonly  in  the  latitude  of  England,  but  frequently  in 
the  south.  It  is  probably  due  to  the  reflection  of  sunlight 
from  millions  of  meteorites  which  have  not  yet  been  swal- 
lowed by  the  sun. 

Again,  if  a  stream  of  meteorites  moves  in  a  very  elliptic 
orbit,  at  one  part  of  its  course  it  passes  near  the  sun.  In 
this  part  of  its  orbit  the  flight  is  packed  into  a  smaller  space 
than  before,  so  that  collisions  are  largely  multiplied.  More- 


182  DARWIN 

over,  the  flight  dashes  through  a  region  thickly  peopled 
with  the  meteorites  which  make  the  zodiacal  light.  It  has 
been  proved  that  there  is  an  intimate  relationship  between 
comets  and  flights  of  meteorites,  and  Mr.  Lockyer  sug- 
gests that  the  luminosity  of  comets  is  caused  jointly  by  the 
collisions  internal  to  the  flight  of  stones,  and  by  those 
which  occur  as  the  flight  ploughs  its  way  through  the  zodi- 
acal light. 

But  meteorites  are  still  frequent  far  outside  of  the  zodi- 
acal light,  although  there  may  not  be  enough  to  reflect 
sunlight  to  a  visible  degree.  Of  this  we  have  familiar  evi- 
dence in  the  shooting  star. 

The  orbits  of  several  streams  of  meteorites  are  known, 
and  each  year,  as  on  certain  days  the  earth  crosses  those 
orbits,  their  existence  is  proved  by  volleys  of  falling  stars, 
which  emanate  from  known  radiant  points  in  the  heavens. 

But  these  are  the  dregs  and  sawdust  of  the  solar  system, 
and  merely  serve  to  give  us  a  memento  of  the  myriads 
which  existed  in  early  days,  before  the  sun  and  the  planets 
and  their  satellites  were  born. 

In  this  paper  I  have  attempted  to  touch  on  only  a  few 
points  in  a  large  subject.  The  attempt  to  reconstruct  the 
history  of  stars  and  planets  involves  ideas  grand  in  them- 
selves; but  the  events  to  be  recorded  in  that  history  relate 
to  a  past  so  remote  that  our  conclusions  can  not  but  be 
speculative.  Thus  the  value  of  the  investigation  of  which 
I  have  given  an  account  will  appear  very  different  to  dif- 
ferent minds.  To  some  men  of  science  it  will  stand  con- 
demned as  altogether  too  speculative;  others  will  think  that 
it  is  better  to  risk  error  in  the  chance  of  winning  truth.  To 
me,  at  least,  it  seems  that  the  line  of  thought  flows  in  a 
true  channel;  that  it  may  help  to  give  a  meaning  to  the 
observations  of  the  astronomer  and  of  the  spectroscopist; 
and  that  many  interesting  problems  may  perhaps  be  solved 
with  sufficient  completeness  to  throw  further  light  on  the 
evolution  of  nebulae  and  of  planetary  systems. 


MAGNITUDE  OF  THE  SOLAR 
SYSTEM 


BY 

WILLIAM   HARKNESS 


MAGNITUDE   OF  THE    SOLAR    SYSTEM1 

NATURE  may  be  studied  in  two  widely  different 
ways.  On  the  one  hand  we  may  employ  a  powerful 
microscope  which  will  render  visible  the  minutest 
forms,  and  limit  our  field  of  view  to  an  infinitesimal  frac- 
tion of  an  inch  situated  within  a  foot  of  our  own  noses;  or, 
on  the  other  hand,  we  may  occupy  some  commanding  posi- 
tion, and  from  thence,  aided  perhaps  by  a  telescope,  we 
may  obtain  a  comprehensive  view  of  an  extensive  region. 
The  first  method  is  that  of  the  specialist,  the  second  is  that 
of  the  philosopher,  but  both  are  necessary  for  an  adequate 
understanding  of  Nature.  The  one  has  brought  us  knowl- 
edge wherewith  to  defend  ourselves  against  bacteria  and 
microbes,  which  are  among  the  most  deadly  enemies  of 
mankind,  and  the  other  has  made  us  acquainted  with  the 
great  laws  of  matter  and  force  upon  which  rests  the  whole 
fabric  of  science.  All  Nature  is  one,  but  for  convenience 
of  classification  we  have  divided  our  knowledge  into  a 
number  of  sciences  which  we  usually  regard  as  quite  dis- 
tinct from  each  other.  Along  certain  lines,  or,  more  prop- 
erly, in  certain  regions,  these  sciences  necessarily  abut 
on  each  other,  and  just  there  lies  the  weakness  of  the  spe- 
cialist. He  is  like  a  wayfarer  who  always  finds  obstacles  in 
crossing  the  boundaries  between  two  countries,  while  to 
the  traveller  who  gazes  over  them  from  a  commanding 
eminence  the  case  is  quite  different.  If  the  boundary  is 

1  Presidential  address  delivered  before  the  American  Association  for 
the  Advancement  of  Science,  at  its  Brooklyn  meeting,  August  16,  1894. 
Printed  in  "  Astronomy  and  Astro-Physics,"  vol.  xiii,  No.  8;  also  in 
"  American  Journal  of  Science,"  vol.  xlviii,  September,  1894;  and  the 
"  Smithsonian  Report  for  1894." 

185 


HARKNESS 

an  ocean  shore,  there  is  no  mistaking  it;  if  a  broad  river  or 
a  chain  of  mountains,  it  is  still  distinct;  but  if  only  a  line  of 
posts  traced  over  hill  and  dale,  then  it  becomes  lost  in  the 
natural  features  of  the  landscape,  and  the  essential  unity 
of  the  whole  region  is  apparent.  In  that  case  the  bor- 
der-land is  wholly  a  human  conception  of  which  Nature 
takes  no  cognizance,  and  so  it  is  with  the  scientific  bor- 
der-land to  which  I  propose  to  invite  your  attention  this 
evening. 

To  the  popular  mind  there  are  no  two  sciences  further 
apart  than  astronomy  and  geology.  The  one  treats  of  the 
structure  and  mineral  constitution  of  our  earth,  the  causes 
of  its  physical  features  and  its  history,  while  the  other  treats 
of  the  celestial  bodies,  their  magnitudes,  motions,  distances, 
periods  of  revolution,  eclipses,  order,  and  of  the  causes  of 
their  various  phenomena.  And  yet  many,  perhaps  I  may 
even  say  most,  of  the  apparent  motions  of  the  heavenly 
bodies  are  merely  reflections  of  the  motions  of  the  earth, 
and  in  studying  them  we  are  really  studying  it.  Further- 
more, precession,  nutation,  and  the  phenomena  of  the  tides 
depend  largely  upon  the  internal  structure  of  the  earth,  and 
there  astronomy  and  geology  merge  into  each  other. 
Nevertheless,  the  methods  of  the  two  sciences  are  widely 
different,  most  astronomical  problems  being  discussed 
quantitatively  by  means  of  rigid  mathematical  formulae, 
while  in  the  vast  majority  of  cases  the  geological  ones  are 
discussed  only  qualitatively,  each  author  contenting  him- 
self with  a  mere  statement  of  what  he  thinks.  With  pre- 
cise data  the  methods  of  astronomy  lead  to  very  exact  re- 
sults, for  mathematics  is  a  mill  which  grinds  exceeding  fine ; 
but,  after  all,  what  comes  out  of  a  mill  depends  wholly  upon 
what  is  put  into  it,  and  if  the  data  are  uncertain,  as  is  the 
case  in  most  cosmological  problems,  there  is  little  to  choose 
between  the  mathematics  of  the  astronomer  and  the  guesses 
of  the  geologist. 

If  we  examine  the  addresses  delivered  by  former  presi- 
dents of  this  association,  and  of  the  sister — perhaps  it  would 
be  nearer  the  truth  to  say  the  parent — association  on  the 


MAGNITUDE   OF   THE   SOLAR   SYSTEM  187 

other  side  of  the  Atlantic,  we  shall  find  that  they  have  gen- 
erally dealt  either  with  the  recent  advances  in  some  broad 
field  of  science,  or  else  with  the  development  of  some  spe- 
cial subject.  This  evening  I  propose  to  adopt  the  latter 
course,  and  I  shall  invite  your  attention  to  the  present  con- 
dition of  our  knowledge  respecting  the  magnitude  of  the 
solar  system;  but  in  so  doing  it  will  be  necessary  to  intro- 
duce some  considerations  derived  from  laboratory  experi- 
ments upon  the  luminiferous  ether,  others  derived  from 
experiments  upon  ponderable  matter,  and  still  others  relat- 
ing both  to  the  surface  phenomena  and  to  the  internal 
structure  of  the  earth,  and  thus  we  shall  deal  largely  with 
the  border-land  where  astronomy,  physics,  and  geology 
merge  into  each  other. 

The  relative  distances  of  the  various  bodies  which  com- 
pose the  solar  system  can  be  determined  to  a  considerable 
degree  of  approximation  with  very  crude  instruments  as 
soon  as  the  true  plan  of  the  system  becomes  known,  and 
that  plan  was  taught  by  Pythagoras  more  than  five  hun- 
dred years  before  Christ.  It  must  have  been  known  to  the 
Egyptians  and  Chaldeans  still  earlier,  if  Pythagoras  really 
acquired  his  knowledge  of  astronomy  from  them,  as  is 
affirmed  by  some  of  the  ancient  writers,  but  on  that  point 
there  is  no  certainty.  In  public  Pythagoras  seemingly  ac- 
cepted the  current  belief  of  his  time,  which  made  the  earth 
the  centre  of  the  universe,  but  to  his  own  chosen  disciples 
he  communicated  the  true  doctrine  that  the  sun  occupies 
the  centre  of  the  solar  system  and  that  the  earth  is  only  one 
of  the  planets  revolving  around  it.  Like  all  the  world's 
greatest  sages,  he  seems  to  have  taught  only  orally.  A 
century  elapsed  before  his  doctrines  were  reduced  to  writ- 
ing by  Philolaus  of  Crotona,  and  it  was  still  later  before  they 
were  taught  in  public  for  the  first  time  by  Hicetas,  or  as  he 
is  sometimes  called  Nicetas,  of  Syracuse.  Then  the  familiar 
cry  of  impiety  was  raised,  and  the  Pythagorean  system  was 
eventually  suppressed  by  that  now  called  the  Ptolemaic, 
which  held  the  field  until  it  was  overthrown  by  Copernicus 
almost  two  thousand  years  later.  Pliny  tells  us  that  Pythag- 


1 88  HARKNESS 

oras  believed  the  distances  to  the  sun  and  moon  to  be, 
respectively,  252,000  and  12,600  stadia,  or,  taking  the 
stadium  at  625  feet,  29,837  and  1,492  English  miles;  but 
there  is  no  record  of  the  method  by  which  these  numbers 
were  ascertained. 

After  the  relative  distances  of  the  various  planets  are 
known,  it  only  remains  to  determine  the  scale  of  the  system, 
for  which  purpose  the  distance  between  any  two  planets 
suffices.  We  know  little  about  the  early  history  of  the  sub- 
ject, but  it  is  clear  that  the  primitive  astronomers  must  have 
found  the  quantities  to  be  measured  too  small  for  detection 
with  their  instruments,  and  even  in  modern  times  the  prob- 
lem has  proved  to  be  an  extremely  difficult  one.  Aris- 
tarchus,  of  Samos,  who  flourished  about  270  B.  c.,  seems 
to  have  been  the  first  to  attack  it  in  a  scientific  manner. 
Stated  in  modern  language,  his  reasoning  was  that  when 
the  moon  is  exactly  half  full  the  earth  and  sun,  as  seen 
from  its  centre,  must  make  a  right  angle  with  each  other, 
and  by  measuring  the  angle  between  the  sun  and  moon, 
as  seen  from  the  earth  at  that  instant,  all  the  angles  of  the 
triangle  joining  the  earth,  sun,  and  moon  would  become 
known,  and  thus  the  ratio  of  the  distance  of  the  sun  to  the 
distance  of  the  moon  would  be  determined.  Although  per- 
fectly correct  in  theory,  the  difficulty  of  deciding  visually 
upon  the  exact  instant  when  the  moon  is  half  full  is  so 
great  that  it  can  not  be  accurately  done,  even  with  the  most 
powerful  telescopes.  Of  course,  Aristarchus  had  no  tele- 
scope, and  he  does  not  explain  how  he  effected  the  observa- 
tion, but  his  conclusion  was  that  at  the  instant  in  question 
the  distance  between  the  centres  of  the  sun  and  moon  as 
seen  from  the  earth  is  less  than  a  right  angle  by  one  thirtieth 
part  of  the  same.  We  should  now  express  this  by  saying 
that  the  angle  is  87°,  but  Aristarchus  knew  nothing  of  trigo- 
nometry, and  in  order  to  solve  his  triangle  he  had  recourse 
to  an  ingenious  but  long  and  cumbersome  geometrical 
process,  which  has  come  down  to  us,  and  affords  conclusive 
proof  of  the  condition  of  Greek  mathematics  at  that  time. 
His  conclusion  was  that  the  sun  is  nineteen  times  farther 


AS   COPE 
Photogravure  from  a  painting. 


e  sun  and  moon  to  be, 

stadia,  or,  taking  the 

.492  English  miles;  but 

.    by  which  these  numbers 

f  the  various  planets  are 
e  the  scale  of  the  system, 
>n  any  two  planets 
iy  history  of  the  sub- 
orners must  have 
iall  for  detection 

one. 


each  other, 

and  sun  and  moon, 

.nt,  all  the  angles  of  the 

would  become 

n  to  the 


he  most 

'io  tele- 

bserva- 

in  question 

mi  and  moon  as 

,gle  by  one  thirtieth 

express  this  by  saying 

tha  r  knew  nothing  of  trigo- 

non.  ;  iangle  he  had  recourse 

to  an  ii  1  cumbersome  geomet: 

5,  and  affords  conclu 
thematics  at  tli 
eteen  times  far: 


MAGNITUDE  OF  THE   SOLAR  SYSTEM  189 

from  the  earth  than  the  moon,  and  if  we  combine  that  re- 
sult with  the  modern  value  of  the  moon's  parallax — viz., 
3,422.38",  we  obtain  for  the  solar  parallax  180",  which  is 
more  than  twenty  times  too  great. 

The  only  other  method  of  determining  the  solar  parallax 
known  to  the  ancients  was  that  devised  by  Hipparchus 
about  150  B.  c.  It  was  based  on  measuring  the  rate  of  de- 
crease of  the  diameter  of  the  earth's  shadow  cone  by  noting 
the  duration  of  lunar  eclipses,  and  as  the  result  deduced 
from  it  happened  to  be  nearly  the  same  as  that  found  by 
Aristarchus,  substantially  his  value  of  the  parallax  remained 
in  vogue  for  nearly  two  thousand  years,  and  the  discovery 
of  the  telescope  was  required  to  reveal  its  erroneous  char- 
acter. Doubtless  this  persistency  was  due  to  the  extreme 
minuteness  of  the  true  parallax,  which  we  now  know  is  far 
too  small  to  have  been  visible  upon  the  ancient  instruments, 
and  thus  the  supposed  measures  of  it  were  really  nothing 
but  measures  of  their  inaccuracy. 

The  telescope  was  first  pointed  to  the  heavens  by 
Galileo  in  1609,  but  it  needed  a  micrometer  to  convert  it 
into  an  accurate  measuring  instrument,  and  that  did  not 
come  into  being  until  1639,  when  it  was  invented  by  Wil- 
liam Gascoigne.  After  his  death,  in  1644,  his  original  in- 
strument passed  to  Richard  Townley,  who  attached  it  to  a 
fourteen-foot  telescope  at  his  residence  in  Townley,  Lanca- 
shire, England,  where  it  was  used  by  Flamsteed  in  observ- 
ing the  diurnal  parallax  of  Mars  during  its  opposition  in 
1672.  A  description  of  Gascoigne's  micrometer  was  pub- 
lished in  the  "  Philosophical  Transactions  "  in  1667,  and  a 
little  before  that  a  similar  instrument  had  been  invented  by 
Auzout,  in  France,  but  observatories  were  fewer  then  than 
now,  and,  so  far  as  I  know,  J.  D.  Cassini  was  the  only  per- 
son besides  Flamsteed  who  attempted  to  determine  the 
solar  parallax  from  that  opposition  of  Mars.  Foreseeing  the 
importance  of  the  opportunity,  he  had  Richer  despatched  to 
Cayenne  some  months  previously,  and  when  the  opposition 
came  he  effected  two  determinations  of  the  parallax:  one 
being  by  the  diurnal  method,  from  his  own  observations 


190  HARKNESS 

in  Paris,  and  the  other  by  the  meridian  method,  from  ob- 
servations in  France  by  himself,  Romer,  and  Picard,  com- 
bined with  those  of  Richer  at  Cayenne.  This  was  the  tran- 
sition from  the  ancient  instruments  with  open  sights  to  tele- 
scopes armed  with  micrometers,  and  the  result  must  have 
been  little  short  of  stunning  to  the  seventeenth-century 
astronomers,  for  it  caused  the  hoary  and  gigantic  parallax 
of  about  1 80"  to  shrink  incontinently  to  10",  and  thus  ex- 
panded their  conception  of  the  solar  system  to  something 
like  its  true  dimensions.  More  than  fifty  years  previously 
Kepler  had  argued  from  his  ideas  of  the  celestial  harmonies 
that  the  solar  parallax  could  not  exceed  60 ",  and  a  little 
later  Horrocks  had  shown  on  more  scientific  grounds  that 
it  was  probably  as  small  as  14";  but  the  final  death  blow 
to  the  ancient  values — ranging  as  high  as  2!  or  3' — came 
from  these  observations  of  Mars  by  Flamsteed,  Cassini, 
and  Richer. 

Of  course,  the  results  obtained  in  1672  produced  a  keen 
desire  on  the  part  of  astronomers  for  further  evidence  re- 
specting the  true  value  of  the  parallax,  and  as  Mars  comes 
into  a  favourable  position  for  such  investigations  only  at 
intervals  of  about  sixteen  years,  they  had  recourse  to  ob- 
servations of  Mercury  and  Venus.  In  1677  Halley  ob- 
served the  diurnal  parallax  of  Mercury,  and  also  a  transit 
of  that  planet  across  the  sun's  disk,  at  St.  Helena,  and  in 
1 68 1  J.  D.  Cassini  and  Picard  observed  Venus  when  she 
was  on  the  same  parallel  with  the  sun,  but  although  the 
observations  of  Venus  gave  better  results  than  those  of 
Mercury,  neither  of  them  was  conclusive,  and  we  now  know 
that  such  methods  are  inaccurate  even  with  the  powerful  in- 
struments of  the  present  day.  Nevertheless,  Halley's  at- 
tempt by  means  of  the  transit  of  Mercury  ultimately  bore 
fruit  in  the  shape  of  his  celebrated  paper  of  1716,  wherein  he 
showed  the  peculiar  advantages  of  transits  of  Venus  for 
determining  the  solar  parallax.  The  idea  of  utilizing  such 
transits  for  this  purpose  seems  to  have  been  vaguely  con- 
ceived by  James  Gregory,  or  perhaps  even  by  Horrocks, 
but  Halley  was  the  first  to  work  it  out  completely,  and 


MAGNITUDE   OF  THE   SOLAR  SYSTEM  191 

long  after  his  death  his  paper  was  mainly  instrumental  in 
inducing  the  governments  of  Europe  to  undertake  the 
observations  of  the  transits  of  Venus  in  1761  and  1769, 
from  which  our  first  accurate  knowledge  of  the  sun's  dis- 
tance was  obtained. 

Those  who  are  not  familiar  with  practical  astronomy 
may  wonder  why  the  solar  parallax  can  be  got  from  Mars 
and  Venus,  but  not  from  Mercury  or  the  sun  itself.  The 
explanation  depends  upon  two  facts:  Firstly,  the  nearest 
approach  of  these  bodies  to  the  earth  is  for  Mars  33,874,000 
miles,  for  Venus  23,654,000  miles,  for  Mercury  47,935,000 
miles,  and  for  the  sun  91,239,000  miles.  Consequently,  for 
us  Mars  and  Venus  have  very  much  larger  parallaxes  than 
Mercury  or  the  sun,  and  of  course  the  larger  the  parallax 
the  easier  it  is  to  measure.  Secondly,  even  the  largest  of 
these  parallaxes  must  be  determined  within  far  less  than 
one  tenth  of  a  second  of  the  truth,  and  while  that  degree  of 
accuracy  is  possible  in  measuring  short  arcs,  it  is  quite  un- 
attainable in  long  ones.  Hence,  one  of  the  most  essential 
conditions  for  the  successful  measurement  of  parallaxes  is 
that  we  shall  be  able  to  compare  the  place  of  the  near  body 
with  that  of  a  more  distant  one  situated  in  the  same  region 
of  the  sky.  In  the  case  of  Mars  that  can  always  be  done 
by  making  use  of  a  neighbouring  star,  but  when  Venus  is 
near  the  earth  she  is  also  so  close  to  the  sun  that  stars  are 
not  available,  and  consequently  her  parallax  can  be  satis- 
factorily measured  only  when  her  position  can  be  accu- 
rately referred  to  that  of  the  sun,  or,  in  other  words,  only 
during  her  transits  across  the  sun's  disk.  But  even  when 
the  two  bodies  to  be  compared  are  sufficiently  near  each 
other,  we  are  still  embarrassed  by  the  fact  that  it  is  more 
difficult  to  measure  the  distance  between  the  limb  of  a 
planet  and  a  star  or  the  limb  of  the  sun  than  it  is  to 
measure  the  distance  between  two  stars,  and  since  the  dis- 
covery of  so  many  asteroids  that  circumstance  has  led  to 
their  use  for  the  determination  of  the  solar  parallax.  Some' 
of  these  bodies  approach  within  75,230,000  miles  of  the 
earth's  orbit,  and  as  they  look  precisely  like  stars,  the  in- 


I92  HARKNESS 

creased  accuracy  of  pointing  on  them  fully  makes  up  for 
their  greater  distance  as  compared  with  Mars  or  Venus. 

After  the  Copernican  system  of  the  world  and  the  New- 
tonian theory  of  gravitation  were  accepted  it  soon  became 
evident  that  trigonometrical  measurements  of  the  solar 
parallax  might  be  supplemented  by  determinations  based 
on  the  theory  of  gravitation,  and  the  first  attempts  in  that 
direction  were  made  by  Machin  in  1729  and  T.  Mayer  in 
1753.  The  measurement  of  the  velocity  of  light  between 
points  on  the  earth's  surface,  first  effected  by  Fizeau  in 
1849,  opened  up  still  other  possibilities,  and  thus  for  deter- 
mining the  solar  parallax  we  have  at  our  command  no  less 
than  three  entirely  distinct  classes  of  methods,  which  are 
known  respectively  as  the  trigonometrical,  the  gravita- 
tional, and  the  photo-tachymetrical.  We  have  already 
given  a  summary  sketch  of  the  trigonometrical  methods  as 
applied  by  the  ancient  astronomers  to  the  dichotomy  and 
shadow  cone  of  the  moon,  and  by  the  moderns  to  Venus, 
Mars,  and  the  asteroids,  and  we  shall  next  glance  briefly 
at  the  gravitational  and  photo-tachymetrical  methods. 

The  gravitational  results  which  enter  directly  or  indi- 
rectly into  the  solar  parallax  are  six  in  number,  to  wit: 
First,  the  relation  of  the  moon's  mass  to  the  tides;  second, 
the  relation  of  the  moon's  mass  and  parallax  to  the  force 
of  gravity  at  the  earth's  surface;  third,  the  relation  of  the 
solar  parallax  to  the  masses  of  the  earth  and  moon;  fourth, 
the  relation  of  the  solar  and  lunar  parallaxes  to  the  moon's 
mass  and  parallactic  inequality;  fifth,  the  relation  of  the 
solar  and  lunar  parallaxes  to  the  moon's  mass  and  the 
earth's  lunar  inequality;  sixth,  the  relation  of  the  constants 
of  nutation  and  precession  to  the  moon's  parallax. 

Respecting  the  first  of  these  relations  it  is  to  be  re- 
marked that  the  tide-producing  forces  are  the  attractions 
of  the  sun  and  moon  upon  the  waters  of  the  ocean,  and 
from  the  ratio  of  these  attractions  the  moon's  mass  can 
readily  be  determined.  But  unfortunately  the  ratio  of  the 
solar  tides  to  the  lunar  tides  is  affected  both  by  the  depth 
of  the  sea  and  by  the  character  of  the  channels  through 


MAGNITUDE   OF  THE   SOLAR  SYSTEM  193 

which  the  water  flows,  and  for  that  reason  the  observed 
ratio  of  these  tides  requires  multiplication  by  a  correcting 
factor  in  order  to  convert  it  into  the  ratio  of  the  forces. 
The  matter  is  further  complicated  by  this  correcting  factor 
varying  from  port  to  port,  and  in  order  to  get  satisfactory 
results  long  series  of  observations  are  necessary.  The 
labour  of  deriving  the  moon's  mass  in  this  way  was  for- 
merly so  great  that  for  more  than  half  a  century  Laplace's 
determination  from  the  tides  at  Brest  remained  unique;  but 
the  recent  application  of  harmonic  analysis  to  the  data  sup- 
plied by  self-registering  tide  gauges  is  likely  to  yield  abun- 
dant results  in  the  near  future. 

Our  second  gravitational  relation — viz.,  that  connecting 
the  moon's  mass  and  parallax  with  the  force  of  gravity  at 
the  earth's  surface — affords  an  indirect  method  of  deter- 
mining the  moon's  parallax  with  very  great  accuracy  if  the 
computation  is  carefully  made,  and  with  a  fair  approxima- 
tion to  the  truth  even  when  the  data  are  exceedingly  crude. 
To  illustrate  this,  let  us  see  what  could  be  done  with  a  rail- 
road transit  such  as  is  commonly  used  by  surveyors,  a  steel 
tape,  and  a  fairly  good  watch.  Neglecting  small  correc- 
tions due  to  the  flattening  of  the  earth,  the  centrifugal  force 
at  its  surface,  the  eccentricity  of  its  orbit  and  the  mass  of 
the  moon,  the  law  of  gravitation  shows  that  if  we  multiply 
together  the  length  of  the  seconds-pendulum,  the  square  of 
the  radius  of  the  earth,  and  the  square  of  the  length  of  the 
sidereal  month,  divide  the  product  by  four,  and  take  the 
cube  root  of  the  quotient,  the  result  will  be  the  distance 
from  the  earth  to  the  moon.  To  find  the  length  of  the  sec- 
onds-pendulum we  would  rate  the  watch  by  means  of  the 
railroad  transit,  and  then  making  a  pendulum  out  of  a 
spherical  leaden  bullet  suspended  by  a  fine  thread,  we 
would  adjust  the  length  of  the  thread  until  the  pendulum 
made  exactly  three  hundred  vibrations  in  five  minutes  by 
the  watch.  Then,  supposing  the  experiment  to  be  made 
here  or  in  New  York  city,  we  would  find  that  the  distance 
from  the  point  of  suspension  of  the  thread  to  the  centre  of 
the  bullet  was  about  39^  inches,  and  dividing  that  by  the 
13 


HARKNESS 

number  of  inches  in  a  mile — viz.,  63,360 — we  would  have 
for  the  length  of  the  seconds-pendulum  -^^  of  a  mile. 
The  next  step  would  be  to  ascertain  the  radius  of  the  earth, 
and  the  quickest  way  of  doing  so  would  probably  be,  first, 
to  determine  the  latitude  of  some  point  in  New  York  city 
by  means  of  the  railroad  transit ;  next  to  run  a  traverse  sur- 
vey along  the  old  post-road  from  New  York  to  Albany,  and 
finally  to  determine  the  latitude  of  some  point  in  Albany. 
The  traverse  survey  should  surely  be  correct  to  one  part  in 
three  hundred,  and  as  the  distance  between  the  two  cities 
is  about  2°,  the  difference  of  latitude  might  be  determined 
to  about  the  same  percentage  of  accuracy.  In  that  way 
we  would  find  the  length  of  2°  of  latitude  to  be  about  138 
miles,  whence  the  earth's  radius  would  be  3,953  miles.  It 
would  then  only  remain  to  observe  the  time  occupied  by 
the  moon  in  making  a  sidereal  revolution  around  the  earth, 
or,  in  other  words,  the  time  which  she  occupies  in  moving 
from  any  given  star  back  to  the  same  star  again.  By  noting 
that  to  within  one  quarter  of  her  own  diameter  we  would 
soon  find  that  the  time  of  revolution  is  about  27.32  days, 
and  multiplying  that  by  the  number  of  seconds  in  a 
day — viz.,  86,400 — we  would  have  for  the  length  of  the 
sidereal  month  2,360,000  seconds.  With  these  data  the 
computation  would  stand  as  follows:  The  radius  of  the 
earth,  3,953  miles,  multiplied  by  the  length  of  a  sidereal 
month,  2,360,000  seconds,  and  the  product  squared  gives 
87,060,000,000,000,000,000.  Multiplying  that  by  one 
fourth  of  the  length  of  the  seconds-pendulum — viz.,  6^s 
of  a  mile — and  extracting  the  cube  root  of  the  product,  we 
would  get  237,700  miles  for  the  distance  from  the  earth 
to  the  moon,  which  is  only  about  850  miles  less  than  the 
truth,  and  certainly  a  remarkable  result  considering  the 
crudeness  of  the  instruments  by  which  it  might  be  obtained. 
Nevertheless,  when  all  the  conditions  are  rigorously  taken 
into  account  these  data  are  to  be  regarded  as  determining 
the  relation  between  the  moon's  mass  and  parallax,  rather 
than  the  parallax  itself. 

Our  third  gravitational  relation — to  wit,  that  existing 


MAGNITUDE   OF  THE   SOLAR   SYSTEM  195 

between  the  solar  parallax,  the  solar  attractive  force,  and 
the  masses  of  the  earth  and  moon — is  analogous  to  the  rela- 
tion existing  between  the  moon's  mass  and  parallax  and  the 
force  of  gravity  at  the  earth's  surface,  but  it  can  not  be 
applied  in  exactly  the  same  way  on  account  of  our  inability 
to  swing  a  pendulum  on  the  sun.  We  are  therefore  com- 
pelled to  adopt  some  other  method  of  determining  the 
sun's  attractive  force,  and  the  most  available  is  that  which 
consists  in  observing  the  perturbative  action  of  the  earth 
and  moon  upon  our  nearest  planetary  neighbours,  Venus 
and  Mars.  From  this  action  the  law  of  gravitation  enables 
us  to  determine  the  ratio  of  the  sun's  mass  to  the  combined 
masses  of  the  earth  and  moon,  and  then  the  relation  in 
question  furnishes  a  means  of  comparing  the  masses  so 
found  with  trigonometrical  determinations  of  the  solar 
parallax.  Thus  it  appears  that  notwithstanding  necessary 
differences  in  the  methods  of  procedure,  the  analogy  be- 
tween the  second  and  third  gravitational  relations  holds  not 
only  with  respect  to  their  theoretical  basis,  but  also  in 
their  practical  application,  the  one  being  used  to  determine 
the  relation  between  the  mass  of  the  moon  and  its  distance 
from  the  earth,  and  the  other  to  determine  the  relation  be- 
tween the  combined  masses  of  the  earth  and  moon  and 
their  distance  from  the  sun. 

Our  fourth  gravitational  relation  deals  with  the  connec- 
tion between  the  solar  parallax,  the  lunar  parallax,  the 
moon's  mass,  and  the  moon's  parallactic  inequality.  The 
important  quantities  are  here  the  solar  parallax  and  the 
moon's  parallactic  inequality,  and  although  the  derivation 
of  the  complete  expression  for  the  connection  between  them 
is  a  little  complicated,  there  is  no  difficulty  in  getting  a 
general  notion  of  the  forces  involved.  As  the  moon  moves 
around  the  earth  she  is  alternately  without  and  within  the 
earth's  orbit.  When  she  is  without,  the  sun's  attraction  on 
her  acts  with  that  of  the  earth ;  when  she  is  within,  the  two 
attractions  act  in  opposite  directions.  Thus  in  effect  the 
centripetal  force  holding  the  moon  to  the  earth  is  alternately 
increased  and  diminished,  with  the  result  of  elongating  the 


196  HARKNESS 

moon's  orbit  toward  the  suri  and  compressing  it  on  the 
opposite  side.  As  the  variation  of  the  centripetal  force  is 
not  great,  the  change  of  form  of  the  orbit  is  small;  never- 
theless, the  summation  of  the  minute  alternations  thereby 
produced  in  the  moon's  orbital  velocity  suffices  to  put  her 
sometimes  ahead  and  sometimes  behind  her  mean  place  to 
an  extent  which  oscillates  from  a  maximum  to  a  minimum, 
as  the  earth  passes  from  perihelion  to  aphelion,  and  aver- 
ages about  125"  of  arc.  This  perturbation  of  the  moon  is 
known  as  the  parallactic  inequality,  because  it  depends  on 
the  earth's  distance  from  the  sun,  and  can  therefore  be  ex- 
pressed in  terms  of  the  solar  parallax.  Conversely,  the 
solar  parallax  can  be  deduced  from  the  observed  value  of 
the  parallactic  inequality,  but  unfortunately  there  are  great 
practical  difficulties  in  making  the  requisite  observations 
with  a  sufficient  degree  of  accuracy.  Notwithstanding  the 
ever-recurring  talk  about  the  advantages  to  be  obtained 
by  observing  a  small,  well-defined  crater  instead  of  the 
moon's  limb,  astronomers  have  hitherto  found  it  imprac- 
ticable to  use  anything  but  the  limb,  and  the  disadvantage 
of  doing  so,  as  compared  with  observing  a  star,  is  still  fur- 
ther increased  by  the  circumstance  that  in  general  only  one 
limb  can  be  seen  at  a  time,  the  other  being  shrouded  in 
darkness.  If  both  limbs  could  always  be  observed,  we 
should  then  have  a  uniform  system  of  data  for  determining 
the  place  of  the  centre,  but  under  existing  circumstances 
we  are  compelled  to  make  our  observations  half  upon  one 
limb  and  half  upon  the  other,  and  thus  they  involve  all  the 
systematic  errors  which  may  arise  from  the  conditions 
under  which  these  limbs  are  observed,  and  all  the  uncer- 
tainty which  attaches  to  irradiation,  personal  equation,  and 
our  defective  knowledge  of  the  moon's  semidiameter. 

Our  fifth  gravitational  relation  is  that  which  exists  be- 
tween the  solar  parallax,  the  lunar  parallax,  the  moon's 
mass,  and  the  earth's  lunar  inequality.  Strictly  speaking, 
the  moon  does  not  revolve  around  the  earth's  centre,  but 
both  bodies  revolve  around  the  common  centre  of  gravity 
of  the  two.  In  consequence  of  that,  an  irregularity  arises 


MAGNITUDE   OF   THE   SOLAR   SYSTEM  197 

in  the  earth's  orbital  velocity  around  the  sun,  the  common 
centre  of  gravity  moving  in  accordance  with  the  laws  of 
elliptic  motion,  while  the  earth,  on  account  of  its  revolution 
around  that  centre,  undergoes  an  alternate  acceleration  and 
retardation  which  has  for  its  period  a  lunar  month,  and  is 
called  the  lunar  inequality  of  the  earth's  motion.  We  per- 
ceive this  inequality  as  an  oscillation  superposed  on  the 
elliptic  motion  of  the  sun,  and  its  semiamplitude  is  the 
measure  of  the  angle  subtended  at  the  sun  by  the  interval 
between  the  centre  of  the  earth  and  the  common  centre  of 
gravity  of  the  earth  and  moon.  Just  as  an  astronomer  on 
the  moon  might  use  the  radius  of  her  orbit  around  the  earth 
as  a  base  for  measuring  her  distance  from  the  sun,  so  we 
may  use  this  interval  for  the  same  purpose.  We  find  its 
length  in  miles  from  the  equatorial  semidiameter  of  the 
earth,  the  moon's  parallax,  and  the  moon's  mass,  and  thus 
we  have  all  the  data  for  determining  the  solar  parallax  from 
the  inequality  in  question.  In  view  of  the  great  difficulty 
which  has  been  experienced  in  measuring  the  solar  parallax 
itself,  it  may  be  asked,  Why  should  we  attempt  to  deal  with 
the  parallactic  inequality,  which  is  about  twenty-six  per 
cent  smaller?  The  answer  is,  Because  the  latter  is  derived 
from  differences  of  the  sun's  right  ascension,  which  are  fur- 
nished by  the  principal  observatories  in  vast  numbers,  and 
should  give  very  accurate  results  on  account  of  their  be- 
ing made  by  methods  which  insure  freedom  from  constant 
errors.  Nevertheless,  the  sun  is  not  so  well  adapted  for 
precise  observations  as  the  stars,  and  Dr.  Gill  has  recently 
found  that  heliometer  measurements  upon  asteroids  which 
approach  very  near  to  the  earth  yield  values  of  the  paral- 
lactic inequality  superior  to  those  obtained  from  right 
ascensions  of  the  sun. 

Our  sixth  gravitational  relation  is  that  which  exists  be- 
tween the  moon's  parallax  and  the  constants  of  precession 
and  nutation.  Every  particle  of  the  earth  is  attracted  both 
by  the  sun  and  by  the  moon,  but  in  consequence  of  the 
polar  flattening  the  resultant  of  these  attractions  passes  a 
little  to  one  side  of  the  earth's  centre  of  gravity.  Thus  a 


198 


HARKNESS 


couple  is  set  up,  which,  by  its  action  upon  the  rotating 
earth,  causes  the  axis  thereof  to  describe  a  surface  which 
may  be  called  a  fluted  cone,  with  its  apex  at  the  earth's 
centre.  A  top  spinning  with  its  axis  inclined  describes  a 
similar  cone,  except  that  the  flutings  are  absent  and  the 
apex  is  at  the  point  upon  which  the  spinning  occurs.  For 
convenience  of  computation  we  resolve  this  action  into  two 
components,  and  we  name  that  which  produces  the  cone 
the  luni-solar  precession,  and  that  which  produces  the  flut- 
ings the  nutation.  In  this  phenomenon  the  part  played  by 
the  sun  is  comparatively  small,  and  by  eliminating  it  we 
obtain  a  relation  between  the  luni-solar  precession,  the 
nutation,  and  the  moon's  parallax  which  can  be  used  to 
verify  and  correct  the  observed  values  of  these  quantities. 

In  the  preceding  paragraph  we  have  seen  that  the  rela- 
tion between  the  quantities  there  considered  depends 
largely  upon  the  flattening  of  the  earth,  and  thus  we  are 
led  to  inquire  how  and  with  what  degree  of  accuracy  that 
is  determined.  There  are  five  methods — viz.,  one  geodetic, 
one  gravitational,  and  three  astronomical.  The  geodetic 
method  depends  upon  measurements  of  the  length  of  a 
degree  on  various  parts  of  the  earth's  surface;  and  with  the 
data  hitherto  accumulated  it  has  proved  quite  unsatisfac- 
tory. The  gravitational  method  consists  in  determining 
the  length  of  the  seconds-pendulum  over  as  great  a  range 
of  latitude  as  possible,  and  deducing  therefrom  the  ratio 
of  the  earth's  polar  and  equatorial  semidiameters  by  means 
of  Clairaut's  theorem.  The  pendulum  experiments  show 
that  the  earth's  crust  is  less  dense  on  mountain  plateaus 
than  at  the  seacoast,  and  thus  for  the  first  time  we  are 
brought  into  contact  with  geological  considerations.  The 
first  astronomical  method  consists  in  observing  the  moon's 
parallax  from  various  points  on  the  earth's  surface;  and 
as  these  parallaxes  are  nothing  else  than  the  angular  semi- 
diameter  of  the  earth  at  the  respective  points,  as  seen  from 
the  moon,  they  afford  a  direct  measure  of  the  flattening. 
The  second  and  third  astronomical  methods  are  based  upon 
certain  perturbations  of  the  moon  which  depend  upon  the 


MAGNITUDE   OF   THE   SOLAR   SYSTEM  199 

figure  of  the  earth,  and  should  give  extremely  accurate 
results;  but  unfortunately  very  great  difficulties  oppose 
themselves  to  the  exact  measurement  of  the  perturbations. 
There  is  also  an  astronomico-geological  method  which 
can  not  yet  be  regarded  as  conclusive  on  account  of  our 
lack  of  knowledge  respecting  the  law  of  density  which  pre- 
vails in  the  interior  of  the  earth.  It  is  based  upon  the 
fact  that  a  certain  function  of  the  earth's  moments  of  inertia 
can  be  determined  from  the  observed  values  of  the  coeffi- 
cients of  precession  and  nutation,  and  could  also  be  deter- 
mined from  the  figure  and  dimensions  of  the  earth  if  we 
knew  the  exact  distribution  of  matter  in  its  interior.  Our 
present  knowledge  on  that  subject  is  limited  to  a  super- 
ficial layer  not  more  than  ten  miles  thick,  but  it  is  usual 
to  assume  that  the  deeper  matter  is  distributed,  according 
to  Lagrange's  law,  and  then  by  writing  the  function  in 
question  in  a  form  which  leaves  the  flattening  indetermi- 
nate, and  equating  the  expression  so  found  to  the  value 
given  by  the  precession  and  nutation,  we  readily  obtain  the 
flattening.  As  yet  these  methods  do  not  give  consistent 
results,  and  so  long  as  serious  discrepancies  remain  be- 
tween them  there  can  be  no  security  that  we  have  arrived 
at  the  truth. 

It  should  be  remarked  that  in  order  to  compute  the 
function  of  the  earth's  moments  of  inertia  which  we  have 
just  been  considering,  we  require  not  only  the  figure  and 
dimensions  of  the  earth  and  the  law  of  distribution  of  den- 
sity in  its  interior,  but  also  its  mean  and  surface  densities. 
The  experiments  for  determining  the  mean  density  have 
consisted  in  comparing  the  earth's  attraction  with  the 
attraction  either  of  a  mountain  or  of  a  known  thickness  of 
the  earth's  crust  or  of  a  known  mass  of  metal.  In  the  case 
of  mountains  the  comparisons  have  been  made  with  plumb 
lines  and  pendulums;  in  the  case  of  known  layers  of  the 
earth's  crust  they  have  been  made  by  swinging  pendu- 
lums at  the  surface  and  down  in  mines;  and  in  the  case 
of  known  masses  of  metal  they  have  been  made  with  tor- 
sion balances,  fine  chemical  balances,  and  pendulums.  The 


200  HARKNESS 

surface  density  results  from  a  study  of  the  materials  com- 
posing the  earth's  crust,  but  notwithstanding  the  appar- 
ent simplicity  of  that  process  it  is  doubtful  if  we  have  yet 
attained  as  accurate  a  result  as  in  the  case  of  the  mean 
density. 

Before  quitting  this  part  of  our  subject  it  is  important 
to  point  out  that  the  luni-solar  precession  can  not  be  directly 
observed,  but  must  be  derived  from  the  general  precession. 
The  former  of  these  qualities  depends  only  upon  the  action 
of  the  sun  and  moon,  while  the  latter  is  affected  in  addition 
by  the  action  of  all  the  planets,  and  to  ascertain  what  that 
is  we  must  determine  their  masses.  The  methods  of  doing 
so  fall  into  two  great  classes,  according  as  the  planets  dealt 
with  have  or  have  not  satellites.  The  most  favourable  case 
is  that  in  which  one  or  more  satellites  are  present,  because 
the  mass  of  the  primary  follows  immediately  from  their 
distances  and  revolution  times;  but  even  then  there  is  a 
difficulty  in  the  way  of  obtaining  very  exact  results.  By 
extending  the  observations  over  sufficiently  long  periods 
the  revolution  times  may  be  ascertained  with  any  desired 
degree  of  accuracy;  but  all  measurements  of  the  distance  of 
a  satellite  from  its  primary  are  affected  by  personal  equa- 
tion, which  we  can  not  be  sure  of  completely  eliminating, 
and  thus  a  considerable  margin  of  uncertainty  is  brought 
into  the  masses.  In  the  cases  of  Mercury  and  Venus, 
which  have  no  satellites,  and  to  a  certain  extent  in  the  case 
of  the  earth  also,  the  only  available  way  of  ascertaining  the 
masses  is  from  the  perturbations  produced  by  the  action  of 
the  various  planets  on  each  other.  These  perturbations  are 
of  two  kinds,  periodic  and  secular.  When  sufficient  data 
have  been  accumulated  for  the  exact  determination  of  the 
secular  perturbations  they  will  give  the  best  results,  but  as 
yet  it  remains  advantageous  to  employ  the  periodic  pertur- 
bations also. 

Passing  now  to  the  photo-tachymetrical  methods,  we 
have  first  to  glance  briefly  at  the  mechanical  appliances  by 
which  the  tremendous  velocity  of  light  has  been  success- 
fully measured.  They  are  of  the  simplest  possible  char- 


MAGNITUDE   OF   THE   SOLAR   SYSTEM  20 1 

acter,  and  are  based  either  upon  a  toothed  wheel  or  upon 
a  revolving  mirror. 

The  toothed-wheel  method  was  first  used  by  Fizeau,  in 
1849.  To  understand  its  operation,  imagine  a  gun-barrel 
with  a  toothed  wheel  revolving  at  right  angles  to  its  muzzle 
in  such  a  way  that  the  barrel  is  alternately  closed  and  opened 
as  the  teeth  and  the  spaces  between  them  pass  before  it. 
Then,  with  the  wheel  in  rapid  motion,  at  the  instant  when 
a  space  is  opposite  the  muzzle  let  a  ball  be  fired.  It  will 
pass  out  freely,  and  after  traversing  a  certain  distance  let 
it  strike  an  elastic  cushion  and  be  reflected  back  upon  its 
own  path.  When  it  reaches  the  wheel,  if  it  hits  a  space  it 
will  return  into  the  gun-barrel,  but  if  it  hits  a  tooth  it  will 
be  stopped.  Examining  the  matter  a  little  more  closely, 
we  see  that,  as  the  ball  requires  a  certain  time  to  go  and 
return,  if  during  that  time  the  wheel  moves  through  an  odd 
multiple  of  the  angle  between  a  space  and  a  tooth  the  ball 
will  be  stopped,  while  if  it  moves  through  an  even  multiple 
of  that  angle  the  ball  will  return  into  the  barrel.  Now, 
imagine  the  gun-barrel,  the  ball,  and  the  elastic  cushion  to 
be  replaced,  respectively,  by  a  telescope,  a  light-wave,  and 
a  mirror.  Then  if  the  wheel  moved  at  such  a  speed  that 
the  returning  light-wave  struck  against  the  tooth  following 
the  space  through  which  it  issued,  to  an  eye  looking  into 
the  telescope  all  would  be  darkness.  If  the  wheel  moved  a 
little  faster  and  the  returning  light-wave  passed  through  the 
space  succeeding  that  through  which  it  issued,  the  eye  at 
the  telescope  would  perceive  a  flash  of  light,  and  if  the 
speed  was  continuously  increased  a  continual  succession  of 
eclipses  and  illuminations  would  follow  each  other  accord- 
ing as  the  returning  light  was  stopped  against  a  tooth  or 
passed  through  a  space  farther  and  farther  behind  that 
through  which  it  issued.  Under  these  conditions  the  time 
occupied  by  the  light  in  traversing  the  space  from  the  wheel 
to  the  mirror  and  back  again  would  evidently  be  the  same 
as  the  time  required  by  the  wheel  to  revolve  through  the 
angle  between  the  space  through  which  the  light  issued 
and  that  through  which  it  returned,  and  thus  the  velocity 


202  HARKNESS 

of  light  would  become  known  from  the  distance  between 
the  telescope  and  the  mirror,  together  with  the  speed  of 
the  wheel.  Of  course  the  longer  the  distance  traversed  and 
the  greater  the  velocity  of  the  wheel  the  more  accurate 
would  be  the  result. 

The  revolving-mirror  method  was  first  used  by  Fou- 
cault  in  1862.  Conceive  the  toothed  wheel  of  Fizeau's  ap- 
paratus to  be  replaced  by  a  mirror  attached  to  a  vertical  axis 
and  capable  of  being  put  into  rapid  rotation.  Then  it  will 
be  possible  so  to  arrange  the  apparatus  that  light  issuing 
from  the  telescope  shall  strike  the  movable  mirror  and  be 
reflected  to  the  distant  mirror,  whence  it  will  be  returned  to 
the  movable  mirror  again,  and  being  thrown  back  into  the 
telescope  will  appear  as  a  star  in  the  centre  of  the  field  of 
view.  That  adjustment  being  made,  if  the  mirror  were 
caused  to  revolve  at  a  speed  of  some  hundred  turns  per 
second  it  would  move  through  an  appreciable  angle  while 
the  light  was  passing  from  it  to  the  distant  mirror  and  back 
again,  and,  in  accordance  with  the  laws  of  reflection,  the  star 
in  the  field  of  the  telescope  would  move  from  the  centre  by 
twice  the  angle  through  which  the  mirror  had  turned. 
Thus  the  deviation  of  the  star  from  the  centre  of  the  field 
would  measure  the  angle  through  which  the  mirror  turned 
during  the  time  occupied  by  light  in  passing  twice  over  the 
interval  between  the  fixed  and  revolving  mirrors,  and  from 
the  magnitude  of  that  angle,  together  with  the  known 
speed  of  the  mirror,  the  velocity  of  the  light  could  be 
calculated. 

In  applying  either  of  these  methods  the  resulting  ve- 
locity is  that  of  light  when  traversing  the  earth's  atmos- 
phere, but  what  we  want  is  its  velocity  in  space,  which  we 
suppose  to  be  destitute  of  ponderable  material,  and  in  order 
to  obtain  that  the  velocity  in  the  atmosphere  must  be  mul- 
tiplied by  the  refractive  index  of  air.  The  correct  velocity 
so  obtained  can  then  be  used  to  find  the  solar  parallax, 
either  from  the  time  required  by  light  to  traverse  the  semi- 
diameter  of  the  earth's  orbit,  or  from  the  ratio  of  the  ve- 
locity of  light  to  the  orbital  velocity  of  the  earth. 


MAGNITUDE   OF  THE   SOLAR   SYSTEM 


203 


Any  periodic  correction  which  occurs  in  computing  the 
place  of  a  heavenly  body  or  the  time  of  a  celestial  phe- 
nomenon is  called  by  astronomers  an  equation,  and  as  the 
time  required  by  light  to  traverse  the  semidiameter  of  the 
earth's  orbit  first  presented  itself  in  the  guise  of  a  correction 
to  the  computed  times  of  the  eclipses  of  Jupiter's  satellites, 
it  has  received  the  name  of  the  light  equation.  The  earth's 
orbit  being  interior  to  that  of  Jupiter,  and  both  having  the 
sun  for  their  centre,  it  is  evident  that  the  distances  between 
the  two  planets  must  vary  from  the  sum  to  the  difference  of 
the  radii  of  their  respective  orbits,  and  the  time  required 
by  light  to  travel  from  one  planet  to  the  other  must  vary 
proportionately.  Consequently,  if  the  observed  times  of 
the  eclipses  of  Jupiter's  satellites  are  compared  with  the 
times  computed  upon  the  assumption  that  the  two  planets 
are  always  separated  by  their  mean  distance,  it  will  be  found 
that  the  eclipses  occur  too  early  when  the  earth  is  at  less 
than  its  mean  distance  from  Jupiter,  and  too  late  when  it 
is  farther  off,  and  from  large  numbers  of  such  observations 
the  value  of  the  light  equation  has  been  deduced. 

The  combination  of  the  motion  of  light  through  our 
atmosphere  with  the  orbital  motion  of  the  earth  gives  rise 
to  the  annual  aberration,  all  the  phases  of  which  are  com- 
puted from  its  maximum  value,  commonly  called  the  con- 
stant of  aberration.  There  is  also  a  diurnal  aberration  due 
to  the  rotation  of  the  earth  on  its  axis,  but  that  is  quite 
small  and  does  not  concern  us  this  evening.  When  aberra- 
tion was  discovered  the  corpuscular  theory  of  light  was  in 
vogue,  and  it  offered  a  charmingly  simple  explanation  of 
the  whole  phenomenon.  The  hypothetical  light  corpuscles 
impinging  upon  the  earth  were  thought  to  behave  pre- 
cisely like  the  drops  in  a  shower  of  rain,  and  you  all  know 
that  their  apparent  direction  is  affected  by  any  motion  on 
the  part  of  the  observer.  In  a  calm  day,  when  the  drops 
are  falling  perpendicularly,  a  man  standing  still  holds  his 
umbrella  directly  over  his  head,  but  as  soon  as  he  begins 
to  move  forward  he  inclines  his  umbrella  in  the  same  direc- 
tion, and  the  more  rapidly  he  moves  the  greater  must  be 


204 


HARKNESS 


its  inclination  in  order  to  meet  the  descending  shower. 
Similarly,  the  apparent  direction  of  oncoming  light  cor- 
puscles would  be  affected  by  the  orbital  motion  of  the  earth, 
so  that  in  effect  it  would  always  be  the  resultant  arising 
from  combining  the  motion  of  the  light  with  a  motion  equal 
and  opposite  to  that  of  the  earth.  But  since  the  falsity  of 
the  corpuscular  theory  has  been  proved  that  explanation 
is  no  longer  tenable,  and  as  yet  we  have  not  been  able  to 
replace  it  with  anything  equally  satisfactory  based  on  the 
now  universally  accepted  undulatory  theory.  In  accord- 
ance with  the  latter  theory  we  must  conceive  the  earth  as 
ploughing  its  way  through  the  ether,  and  the  point  which 
has  hitherto  baffled  us  is  whether  or  not  in  so  doing  it 
produces  any  disturbance  of  the  ether  which  affects  the 
aberration.  In  our  present  ignorance  on  that  point  we 
can  only  say  that  the  aberration  constant  is  certainly  very 
nearly  equal  to  the  ratio  of  the  earth's  orbital  velocity  to 
the  velocity  of  light,  but  we  can  not  affirm  that  it  is  rigor- 
ously so. 

The  luminiferous  ether  was  invented  to  account  for  the 
phenomena  of  light,  and  for  two  hundred  years  it  was  not 
suspected  of  having  any  other  function.  The  emission 
theory  postulated  only  the  corpuscles  which  constitute  light 
itself,  but  the  undulatory  theory  fills  all  space  with  an  im- 
ponderable substance  possessing  properties  even  more  re- 
markable than  those  of  ordinary  matter,  and  to  some  of  the 
acutest  intellects  the  magnitude  of  this  idea  has  proved  an 
almost  insuperable  objection  against  the  whole  theory.  So 
late  as  1862  Sir  David  Brewster,  who  had  gained  a  world- 
wide reputation  by  his  optical  researches,  expressed  himself 
as  staggered  by  the  notion  of  filling  all  space  with  some 
substance  merely  to  enable  a  little  twinkling  star  to  send 
its  light  to  us;  but  not  long  after  Clerk  Maxwell  removed 
that  difficulty  by  a  discovery  coextensive  with  the  undu- 
latory theory  itself.  Since  1845,  when  Faraday  first  per- 
formed his  celebrated  experiment  of  magnetizing  a  ray  of 
light,  the  idea  that  electricity  is  a  phenomenon  of  the  ether 
had  been  steadily  growing,  until  at  last  Maxwell  perceived 


MAGNITUDE   OF  THE   SOLAR   SYSTEM  205 

that  if  such  were  the  fact  the  rate  of  propagation  of  an 
electro-magnetic  wave  must  be  the  same  as  the  velocity  of 
light.  At  that  time  no  one  knew  how  to  generate  such 
waves,  but  Maxwell's  theory  showed  him  that  their  velocity 
must  be  equal  to  the  number  of  electric  units  of  quantity 
in  the  electro-magnet  unit,  and  careful  experiments  soon 
proved  that  that  is  the  velocity  of  light.  Thus  it  was  put 
almost  beyond  the  possibility  of  doubt  that  the  ether  gives 
rise  to  the  phenomena  of  electricity  and  magnetism,  as  well 
as  to  those  of  light,  and  perhaps  it  may  even  be  concerned 
in  the  production  of  gravitation  itself.  What  could  be  ap- 
parently more  remote  than  these  electric  quantities  and  the 
solar  parallax?  And  yet  we  have  here  a  relation  between 
them,  but  we  make  no  use  of  it  because  as  yet  the  same 
relation  can  be  far  more  accurately  determined  from  ex- 
periments upon  the  velocity  of  light. 

Now,  let  us  recall  the  quantities  and  methods  of  ob- 
servation which  we  have  found  to  be  involved,  either  di- 
rectly or  indirectly,  with  the  solar  parallax.  They  are,  the 
solar  parallax,  obtained  from  transits  of  Venus,  oppositions 
of  Mars,  and  oppositions  of  certain  asteroids;  the  lunar 
parallax,  found  both  directly  and  from  measurements  of 
the  force  of  gravity  at  the  earth's  surface;  the  con- 
stants of  precession,  nutation,  and  aberration,  obtained 
from  observations  of  the  stars;  the  parallactic  inequality 
of  the  moon;  the  lunar  inequality  of  the  earth,  usually 
obtained  from  observations  of  the  sun,  but  recently 
found  from  heliometer  observations  of  certain  asteroids; 
the  mass  of  the  earth,  found  from  the  solar  parallax 
and  also  from  the  periodic  and  secular  perturbations  of 
Venus  and  Mars;  the  mass  of  the  moon,  found  from  the 
lunar  inequality  of  the  earth  and  also  from  the  ratio  of  the 
solar  and  lunar  components  of  the  ocean  tides;  the  masses 
of  all  the  planets  obtained  from  observations  of  their  satel- 
lites whenever  possible,  and  when  no  satellites  exist,  then 
from  observations  of  their  mutual  perturbations,  both  peri- 
odic and  secular;  the  velocity  of  light,  obtained  from  ex- 
periments with  revolving  mirrors  and  toothed  wheels, 


206  HARKNESS 

together  with  laboratory  determinations  of  the  index  of 
refraction  of  atmospheric  air;  the  light  equation,  obtained 
from  observations  of  the  eclipses  of  Jupiter's  satellites;  the 
figure  of  the  earth,  obtained  from  geodetic  triangulations, 
measurements  of  the  length  of  the  seconds-pendulum  in 
various  latitudes,  and  observations  of  certain  perturbations 
of  the  moon;  the  mean  density  of  the  earth,  obtained  from 
measurements  of  the  attractions  of  mountains,  from  pen- 
dulum experiments  in  mines,  and  from  experiments  on  the 
attraction  of  known  masses  of  matter  made  either  with 
torsion  balances  or  with  the  most  delicate  chemical  bal- 
ances; the  surface  density  of  the  earth,  obtained  from  geo- 
logical examinations  of  the  surface  strata;  and,  lastly,  the 
law  of  distribution  of  density  in  the  interior  of  the  earth, 
which  in  the  present  state  of  geological  knowledge  we  can 
do  little  more  than  guess  at. 

Here,  then,  we  have  a  large  group  of  astronomical,  geo- 
detic, geological,  and  physical  quantities  which  must  all  be 
considered  in  finding  the  solar  parallax,  and  which  are  all 
so  entangled  with  each  other  that  no  one  of  them  can  be 
varied  without  affecting  all  the  rest.  It  is  therefore  im- 
possible to  make  an  accurate  determination  of  any  one  of 
them  apart  from  the  remainder  of  the  group,  and  thus  we 
are  driven  to  the  conclusion  that  they  must  all  be  deter- 
mined simultaneously.  Such  has  not  been  the  practice  of 
astronomers  in  the  past,  but  it  is  the  method  to  which 
they  must  inevitably  resort  in  the  future.  A  cursory  glance 
at  an  analogous  problem  occurring  in  geodesy  may  be  in- 
structive. When  a  country  is  covered  with  a  net  of  tri- 
angles it  is  always  found  that  the  observed  angles  are  sub- 
ject to  a  certain  amount  of  error,  and  a  century  ago  it  was 
the  habit  to  correct  the  angles  in  each  triangle  without 
much  regard  to  the  effect  upon  adjacent  triangles.  Conse- 
quently the  adjustment  of  the  errors  was  imperfect,  and  in 
computing  the  interval  between  any  two  distant  points  the 
result  would  vary  somewhat  with  the  triangles  used  in 
the  computation — that  is,  if  one  computation  was  made 
through  a  chain  of  triangles  running  around  on  the  right- 


MAGNITUDE   OF   THE   SOLAR   SYSTEM  207 

hand  side,  another  through  a  chain  of  triangles  running 
straight  between  the  two  points,  and  a  third  through  a 
chain  of  triangles  running  around  on  the  left-hand  side,  the 
results  were  usually  all  different.  At  that  time  things  were 
less  highly  specialized  than  now,  and  all  geodetic  opera- 
tions were  yet  in  the  hands  of  first-rate  astronomers,  who 
soon  devised  processes  for  overcoming  the  difficulty.  They 
imagined  every  observed  angle  to  be  subject  to  a  small  cor- 
rection, and  as  these  corrections  were  all  entangled  with 
each  other  through  the  geometrical  conditions  of  the  net, 
by  a  most  ingenious  application  of  the  method  of  least 
squares  they  determined  them  all  simultaneously  in  such 
a  way  as  to  satisfy  the  whole  of  the  geometrical  conditions. 
Thus  the  best  possible  adjustment  was  obtained,  and  no 
matter  what  triangles  were  used  in  passing  from  one  point 
to  another,  the  result  was  always  the  same.  That  method 
is  now  applied  to  every  important  triangulation,  and  its 
omission  would  be  regarded  as  proof  of  incompetency  on 
the  part  of  those  in  charge  of  the  work. 

Now,  let  us  compare  the  conditions  existing  respectively 
in  a  triangulation  net  and  in  the  group  of  quantities  for  the 
determination  of  the  solar  parallax.  In  the  net  every  angle 
is  subject  to  a  small  correction,  and  the  whole  system  of 
corrections  must  be  so  determined  as  to  make  the  sum  of 
their  weighted  squares  a  minimum  and  at  the  same  time 
satisfy  all  the  geometrical  conditions  of  the  net.  Like  the 
triangles,  the  quantities  composing  the  group  from  which 
the  solar  parallax  must  be  determined  are  all  subject  to 
error,  and  therefore  we  must  regard  each  of  them  as  requir- 
ing a  small  correction,  and  all  these  corrections  must  be 
so  determined  as  to  make  the  sum  of  their  weighted  squares 
a  minimum,  and  at  the  same  time  satisfy  every  one  of  the 
equations  expressing  the  relations  between  the  various 
components  of  the  group. 

Thus  it  appears  that  the  method  required  for  adjusting 
the  solar  parallax  and  its  related  constants  is  in  all  respects 
the  same  as  that  which  has  so  long  been  used  for  adjusting 
systems  of  triangulation;  and  as  the  latter  method  was  in- 


208  HARKNESS 

vented  by  astronomers,  it  is  natural  to  inquire,  Why  have 
they  not  applied  it  to  the  fundamental  problem  of  their 
own  science?  The  reasons  are  various,  but  they  may  all  be 
classed  under  two  heads:  First,  an  inveterate  habit  of  over- 
estimating the  accuracy  of  our  own  work  as  compared  with 
that  of  others;  and,  second,  the  unfortunate  effect  of  too 
much  specialization. 

The  prevailing  opinion  certainly  is  that  great  advances 
have  recently  been  made  in  astronomy,  and  so  they  have  in 
the  fields  of  spectral  analysis  and  in  the  measurement  of 
minute  quantities  of  radiant  heat;  but  the  solution  of  the 
vast  majority  of  astronomical  problems  depends  upon  the 
exact  measurement  of  angles,  and  in  that  little  or  no 
progress  has  been  made.  Bradley,  with  his  zenith  sector 
a  hundred  and  fifty  years  ago,  and  Bessel  and  Struve,  with 
their  circles  and  transit  instruments  seventy  years  ago, 
made  observations  not  sensibly  inferior  to  those  of  the 
present  day,  and  indeed  it  would  have  been  surprising  if 
they  had  not  done  so.  The  essentials  for  accurately  deter- 
mining star-places  are  a  skilled  observer,  a  clock,  and  a 
transit  circle,  the  latter  consisting  of  a  telescope,  a  divided 
circle,  and  four  micrometer  microscopes.  Surely  no  one 
will  claim  that  we  have  to-day  any  more  skilful  observers 
than  were  Bessel,  Bradley,  and  Struve,  and  the  only  way 
in  which  we  have  improved  upon  the  telescopes  made  by 
Dollond  one  hundred  and  thirty  years  ago  is  by  increasing 
their  aperture  and  relatively  diminishing  their  focal  dis- 
tance. The  most  famous  dividing  engine  now  in  existence 
was  made  by  the  elder  Repsold  seventy-five  years  ago;  but 
as  the  errors  of  divided  circles  and  their  micrometer  micro- 
scopes are  always  carefully  determined,  the  accuracy  of  the 
measured  angles  is  quite  independent  of  any  small  improve- 
ment in  the  accuracy  of  the  divisions  or  of  the  micrometer 
screws.  Only  in  the  matter  of  clocks  has  there  been  some 
advance,  and  even  that  is  not  very  great.  On  the  whole, 
the  star-places  of  to-day  are  little  better  than  those  of 
seventy-five  years  ago,  but  even  yet  there  is  great  room 
for  improvement.  One  of  the  commonest  applications  of 


MAGNITUDE   OF   THE   SOLAR   SYSTEM 


209 


these  star-places  is  to  the  determination  of  latitude,  but  it 
is  very  doubtful  if  there  is  any  point  on  the  face  of  the 
earth  whose  latitude  is  known  certainly  within  one  tenth 
of  a  second. 

Looking  at  the  question  from  another  point  of  view,  it 
is  notorious  that  the  contact  observations  of  the  transits  of 
Venus  in  1761  and  1769  were  so  discordant  that  from  the 
same  observations  Encke  and  E.  J.  Stone  got  respectively 
for  the  solar  parallax  8.59"  and  8.9 1".  In  1870  no  one 
thought  it  possible  that  there  could  be  any  difficulty  with 
the  contact  observations  of  the  then  approaching  transits 
of  1874  and  1882,  but  we  have  found  from  sad  experience 
that  our  vaunted  modern  instruments  gave  very  little  bet- 
ter results  for  the  last  pair  of  transits  than  our  prede- 
cessors obtained  with  much  cruder  appliances  in  1761 
and  1769. 

The  theory  of  probability  and  uniform  experience  alike 
show  that  the  limit  of  accuracy  attainable  with  any  instru- 
ment is  soon  reached;  and  yet  we  all  know  the  fascination 
which  continually  lures  us  on  in  our  efforts  to  get  better 
results  out  of  the  familiar  telescopes  and  circles  which  have 
constituted  the  standard  equipment  of  observatories  for 
nearly  a  century.  Possibly  these  instruments  may  be  capa- 
ble of  indicating  somewhat  smaller  quantities  than  we  have 
hitherto  succeeded  in  measuring  with  them,  but  their  limit 
can  not  be  far  off,  because  they  already  show  the  disturbing 
effects  of  slight  inequalities  of  temperature  and  other  un- 
controllable causes.  So  far  as  these  effects  are  accidental 
they  eliminate  themselves  from  every  long  series  of  obser- 
vations, but  there  always  remains  a  residuum  of  constant 
error,  perhaps  quite  unsuspected,  which  gives  us  no  end  of 
trouble.  Encke's  value  of  the  solar  parallax  affords  a  fine 
illustration  of  this.  From  the  transits  of  Venus  in  1761 
and  1769  he  found  8.58"  in  1824,  which  he  subsequently 
corrected  to  8.57",  and  for  thirty  years  that  value  was  uni- 
versally accepted.  The  first  objection  to  it  came  from  Han- 
sen  in  1854,  a  second  followed  from  Leverrier  in  1858,  both 
based  upon  facts  connected  with  the  lunar  theory,  and 
14 


2io  HARKNESS 

eventually  it  became  evident  that  Encke's  parallax  was 
about  one  fourth  of  a  second  too  small. 

Now  please  observe  that  Encke's  value  was  obtained 
trigonometrically,  and  its  inaccuracy  was  never  suspected 
until  it  was  revealed  by  gravitational  methods,  which  were 
themselves  in  error  about  one  tenth  of  a  second,  and  re- 
quired subsequent  correction  in  other  ways.  Here,  then, 
was  a  lesson  to  astronomers,  who  are  all  more  or  less 
specialists,  but  it  merely  enforced  the  perfectly  well-known 
principle  that  the  constant  errors  of  any  one  method  are 
accidental  errors  with  respect  to  all  other  methods,  and 
therefore  the  readiest  way  of  eliminating  them  is  by  com- 
bining the  results  from  as  many  different  methods  as  pos- 
sible. However,  the  abler  the  specialist  the  more  certain 
he  is  to  be  blind  to  all  methods  but  his  own,  and  astrono- 
mers have  profited  so  little  by  the  Encke-Hansen-Leverrier 
incident  of  thirty-five  years  ago  that  to-day  they  are  mostly 
divided  into  two  great  parties,  one  of  whom  holds  that 
the  parallax  can  be  best  determined  from  a  combination  of 
the  constant  of  aberration  with  the  velocity  of  light,  and 
the  other  believes  only  in  the  results  of  heliometer  meas- 
urements upon  asteroids.  By  all  means  continue  the  heli- 
ometer measurements  and  do  everything  possible  to  clear 
up  the  mystery  which  now  surrounds  the  constant  of  aber- 
ration; but  why  ignore  the  work  of  predecessors  who  were 
quite  as  able  as  ourselves?  If  it  were  desired  to  determine 
some  one  angle  of  a  triangulation  net  with  special  exact- 
ness, what  would  be  thought  of  a  man  who  attempted  to 
do  so  by  repeated  measurements  of  the  angle  in  question 
while  he  persistently  neglected  to  adjust  the  net?  And  yet 
until  very  recently  astronomers  have  been  doing  precisely 
that  kind  of  thing  with  the  solar  parallax.  I  do  not  think 
there  is  any  exaggeration  in  saying  that  the  trustworthy  ob- 
servations now  on  record  for  the  determination  of  the  nu- 
merous quantities  which  are  functions  of  the  parallax  could 
not  be  duplicated  by  the  most  industrious  astronomer  work- 
ing continuously  for  a  thousand  years.  How,  then,  can  we 
suppose  that  the  result  properly  deducible  from  them  can 


MAGNITUDE  OF  THE   SOLAR  SYSTEM  211 

be  materially  affected  by  anything  that  any  of  us  can  do 
in  a  lifetime  unless  we  are  fortunate  enough  to  invent 
methods  of  measurement  vastly  superior  to  any  hitherto 
imagined?  Probably  the  existing  observations  for  the  de- 
termination of  most  of  these  quantities  are  as  exact  as  any 
that  can  ever  be  made  with  our  present  instruments,  and 
if  they  were  freed  from  constant  errors  they  would  certainly 
give  results  very  near  the  truth.  To  that  end  we  have  only 
to  form  a  system  of  simultaneous  equations  between  all  the 
observed  quantities  and  then  deduce  the  most  probable 
values  of  these  quantities  by  the  method  of  least  squares. 
Perhaps  some  of  you  may  think  that  the  value  so  obtained 
for  the  solar  parallax  would  depend  largely  upon  the  rela- 
tive weights  assigned  to  the  various  quantities,  but  such  is 
not  the  case.  With  almost  any  possible  system  of  weights 
the  solar  parallax  will  come  out  very  nearly  8.809"  ± 
0.0057",  whence  we  have  for  the  mean  distance  between 
the  earth  and  the  sun  92,797,000  miles,  with  a  probable 
error  of  only  59,700  miles;  and  for  the  diameter  of  the 
solar  system,  measured  to  its  outermost  member,  the  planet 
Neptune,  5,578,400,000  miles. 


THE  STABILITY  OF  THE 
SOLAR  SYSTEM 


BY 

ORMSBY  MCKNIGHT  MITCHEL 

(Born  1810;  died  1862) 


THE 
STABILITY   OF  THE   SOLAR  SYSTEM1 

WHEN,  by  the  application  of  a  single  great  law,  the 
mind  had  succeeded  in  resolving  the  difficult 
problems  presented  by  the  motions  of  the  earth 
and  its  satellite,  the  moon,  it  rose  to  the  examination  of 
the  higher  and  more  complicated  questions  of  the  stability 
of  the  entire  sytem  of  planets,  satellites,  and  comets,  which 
are  found  to  pursue  their  courses  round  the  sun.  The 
number  of  bodies  involved  in  this  investigation,  their  mag- 
nitudes and  vast  periods  of  revolution,  their  great  dis- 
tances from  the  observer,  and  the  exceeding  delicacy  of  the 
required  observations,  combined  with  the  high  interest 
which  attaches  itself  to  the  final  results,  have  united  to 
render  this  investigation  the  most  wonderful  which  has  ever 
employed  the  energies  of  the  human  mind. 

To  comprehend  the  dignity  and  importance  of  this 
great  subject,  let  us  rapidly  survey  the  system,  and  moving 
outward  to  its  known  boundaries,  mark  the  number  and 
variety  of  worlds  involved  in  the  investigation.  Beginning, 
then,  at  the  great  centre,  the  grand  controlling  orb,  the  sun, 
we  find  its  magnitude  such  as  greatly  to  exceed  the  com- 
bined masses  of  all  its  attendant  planets.  Indeed,  if  these 
could  all  be  arranged  in  a  straight  line  on  the  same  side 
of  the  sun,  so  that  their  joint  effect  might  be  exerted  on 
that  body,  the  centre  of  gravity  of  the  entire  system,  thus 
located,  would  scarcely  fall  beyond  the  limits  of  the  sun's 
surface.  At  a  mean  distance  of  36,000,000  miles  from  the 
sun  we  meet  the  nearest  planet,  Mercury,  revolving  in  an 

1  From  "  Planetary  and  Stellar  Worlds." 
215 


2l6  MITCHEL 

orbit  of  considerable  eccentricity,  and  completing  its  cir- 
cuit around  the  sun  in  a  period  of  about  88  of  our  days. 
This  world  has  a  diameter  of  only  3,140  miles,  and  is  the 
smallest  of  the  old  planets.  Pursuing  our  journey,  at  a  dis- 
tance of  68,000,000  miles  from  the  sun,  we  cross  the  orbit 
of  the  planet  Venus.  Her  magnitude  is  nearly  equal  to  that 
of  the  earth.  Her  diameter  is  7,700  miles,  and  the  length  of 
her  year  is  nearly  225  of  our  days.  The  next  planet  we 
meet  is  the  earth,  whose  mean  distance  from  the  sun  is 
95,000,000  miles.  The  peculiarities  which  mark  its  move- 
ments and  those  of  its  satellite  have  been  already  dis- 
cussed. Leaving  the  earth,  and  continuing  our  journey 
outward,  we  cross  the  orbit  of  Mars,  at  a  mean  distance 
from  the  sun  of  142,000,000  miles.  This  planet  is  4,100 
miles  in  diameter,  and  performs  its  revolution  around  the 
sun  in  about  687  days,  in  an  orbit  but  little  inclined  to  the 
plane  of  the  ecliptic.  Its  features,  as  we  shall  see  hereafter, 
are  more  nearly  like  those  of  the  earth  than  any  other 
planet.  Beyond  the  orbit  of  Mars,  and  at  a  mean  distance 
from  the  sun  of  about  250,000,000  miles,  we  encounter  a 
group  of  small  planets,  eight  in  number,  presenting  an 
anomaly  in  the  system,  and  entirely  different  from  any- 
thing elsewhere  to  be  found.  These  little  planets  are  called 
asteroids.  Their  orbits  are,  in  general,  more  eccentric,  and 
more  inclined  to  be  ecliptic,  than  those  of  the  other  planets; 
but  the  most  remarkable  fact  is  this,  that  their  orbits  are 
so  nearly  equal  in  size  that,  when  projected  on  a  common 
plane,  they  are  not  inclosed  the  one  within  the  other,  but 
actually  cross  each  other. 

We  shall  return  to  an  examination  of  these  wonderful 
objects  hereafter.  At  a  mean  distance  of  485,000,000  miles 
from  the  sun  we  cross  the  orbit  of  Jupiter,  the  largest  and 
most  magnificent  of  all  the  planets.  His  diameter  is  nearly 
90,000  miles.  He  is  attended  by  four  moons,  and  performs 
his  revolution  round  the  sun  in  a  period  of  nearly  twelve 
years.  Leaving  this  vast  world,  and  continuing  our  jour- 
ney to  a  distance  of  890,000,000  miles  from  the  sun,  we 
cross  the  orbit  of  Saturn,  the  most  wonderful  of  all  the 


THE  STABILITY   OF  THE   SOLAR   SYSTEM          217 

planets.  His  diameter  is  76,068  miles,  and  he  sweeps  round 
the  sun  in  a  period  of  nearly  29^  years.  He  is  sur- 
rounded by  several  broad,  concentric  rings,  and  is  accom- 
panied by  no  fewer  than  seven  satellites  or  moons.  The 
interplanetary  spaces,  we  perceive,  are  rapidly  increasing. 
The  orbit  of  Uranus  is  crossed  at  a  mean  distance  from  the 
sun  of  1,800,000,000  miles.  His  diameter  is  35,000  miles 
and  his  period  of  revolution  amounts  to  rather  more  than 
84  of  our  years.  He  is  attended  by  six  moons,  and  pur- 
sues his  journey  at  a  slower  rate  than  any  of  the  interior 
planets.  Leaving  this  planet,  we  reach  the  boundary  of 
the  planetary  system  at  a  distance  of  about  3,000,000,000 
miles  from  the  sun.  Here  revolves  the  last  discovered 
planet,  Neptune,  attended  by  one,  probably  by  two  moons, 
and  completing  his  vast  circuit  about  the  sun  in  a  period 
of  164  of  our  years.  His  diameter  is  eight  times  greater 
than  the  earth's,  and  he  contains  an  amount  of  matter 
sufficient  to  form  one  hundred  and  twenty-five  worlds  such 
as  ours. 

Here  we  reach  the  known  limit  of  the  planetary  worlds, 
and  standing  at  this  remote  point  and  looking  back  toward 
the  sun,  the  keenest  vision  of  man  could  not  descry  more 
than  one  solitary  planet  along  the  line  we  have  traversed. 
The  distance  is  so  great  that  even  Saturn  and  Jupiter  are 
utterly  invisible,  and  the  sun  itself  has  shrunk  to  be  scarcely 
greater  than  a  fixed  star. 

There  are  certain  great  characteristics  which  distinguish 
this  entire  scheme  of  worlds.  They  are  all  nearly  globular, 
they  all  revolve  on  axes,  their  orbits  are  all  nearly  cir- 
cular, they  all  revolve  in  the  same  direction  around  the  sun, 
the  planes  of  their  orbits  are  but  slightly  inclined  to  each 
other,  and  their  moons  follow  the  same  general  laws.  With 
a  knowledge  of  these  general  facts,  it  is  proposed  to  trace 
the  reciprocal  influences  of  all  these  revolving  worlds,  and 
to  learn,  if  it  be  possible,  whether  this  vast  scheme  has  been 
so  constructed  as  to  endure  while  time  shall  last,  or  whether 
the  elements  of  its  final  dissolution  are  not  contained  within 
itself,  either  causing  the  planets,  one  by  one,  to  drop  into 


2i8  MITCHEL 

the  sun,  or  to  recede  from  this  great  centre,  released  from 
its  influence,  to  pursue  their  lawless  orbits  through  un- 
known regions  of  space. 

Before  proceeding  to  the  investigation  of  the  great 
problem  of  the  stability  of  the  universe,  let  us  examine  how 
far  the  law  of  gravitation  extends  its  influence  over  the 
bodies  which  are  united  in  the  solar  system.  A  broad  and 
distinct  line  must  be  drawn  between  those  phenomena  for 
which  gravitation  must  render  a  satisfactory  account,  and 
those  other  phenomena  for  which  it  is  in  no  wise  respon- 
sible. In  the  solar  system  we  find,  for  example,  that  all 
the  planets  revolve  in  the  same  direction  around  the  sun, 
in  orbits  slightly  elliptical,  and  in  planes  but  little  inclined 
to  each  other.  Neither  of  these  three  peculiarities  is  in  any 
way  traceable  to  the  law  of  gravitation. 

Start  a  planet  in  its  career,  and,  no  matter  what  be  the 
eccentricity  of  its  orbit,  the  direction  of  its  movement,  or 
the  inclination  of  the  plane  in  which  it  pursues  its  journey, 
once  projected,  it  falls  under  the  empire  of  gravitation,  and 
ever  afterward  this  law  is  accountable  for  all  its  movements. 
We  are  not,  therefore,  to  regard  the  remarkable  constitu- 
tion of  the  solar  system  as  a  result  of  any  of  the  known  laws 
of  Nature. 

If  the  sun  were  created,  and  the  planetary  worlds  formed 
and  placed  at  the  disposal  of  a  being  possessed  of  less  than 
infinite  wisdom,  and  he  were  required  so  to  locate  them  in 
space,  and  to  project  them  in  orbits,  such  that  their  revo- 
lutions should  be  eternal,  even  with  the  assistance  of  the 
known  laws  of  motion  and  gravitation,  this  finite  being 
would  fail  to  construct  his  required  system. 

Let  it  be  remembered  that  each  and  every  one  of  these 
bodies  exerts  an  influence  upon  all  the  others.  There  is  no 
isolated  object  in  the  system.  Planet  always  planet,  and 
satellite  bends  the  orbit  of  satellite,  until  the  primitive 
curves  described  lose  the  simplicity  of  their  character,  and 
perturbations  arise,  which  may  end  in  absolute  destruction. 
There  is  no  chance  work  in  the  construction  of  our  mighty 
system.  Every  planet  has  been  weighed  and  poised,  and 


THE   STABILITY  OF  THE   SOLAR   SYSTEM          219 

placed  precisely  where  it  should  be.  If  it  were  possible  to 
drag  Jupiter  from  its  orbit,  and  cause  him  to  change  places 
with  the  planet  Venus,  this  interchange  of  orbits  would  be 
fatal  to  the  stability  of  the  entire  system.  In  contemplat- 
ing the  delicacy  and  complexity  of  the  adjustment  of  the 
planetary  worlds,  the  mind  can  not  fail  to  recognise  the 
fact  that,  in  all  this  intricate  balancing,  there  is  a  higher  ob- 
ject to  be  gained  than  the  mere  perpetuity  of  the  system. 

If  stability  had  been  the  sole  object  it  might  have  been 
gained  by  a  far  simpler  arrangement.  If  God  had  so  con- 
stituted matter  that  the  sun  might  have  attracted  the  plan- 
ets, while  these  should  exert  no  influence  over  each  other — 
that  the  planets  might  have  attracted  their  satellites,  while 
these  were  free  from  their  reciprocal  influences — then,  in- 
deed, a  system  would  have  been  formed  whose  movements 
would  have  been  eternal  and  whose  stability  would  have 
been  independent  of  the  relative  positions  of  the  worlds 
and  the  character  of  their  orbits.  Give  to  them  but  space 
enough  in  which  to  perform  their  revolutions  around  the 
sun,  so  that  no  collisions  might  occur,  freed  from  this  only 
danger,  and  every  planet  and  every  satellite  will  pursue  the 
same  undeviating  track  throughout  the  ceaseless  ages  of 
eternity. 

If  this  statement  be  true,  it  may  be  demanded  why  such 
a  system  was  not  adopted.  It  is  impossible  for  us  to  assign 
all  the  reasons  which  led  to  the  adoption  of  the  present 
complicated  system.  Of  one  thing,  however,  we  are  cer- 
tain: If  God  designed  that  in  the  heavens  his  glory  and 
his  wisdom  should  be  declared,  and  that  in  the  study  of  his 
mighty  works  his  intelligent  creatures  should  rise  higher 
and  higher  toward  his  eternal  throne,  then,  indeed,  has  the 
present  system  been  admirably  constituted  for  the  accom- 
plishment of  this  grand  design.  To  have  acquired  a  knowl- 
edge of  a  system  constituted  of  independent  planets,  free 
from  all  mutual  perturbations,  would  have  required  scarcely 
no  effort  to  the  mind,  when  compared  with  that  put  forth 
in  the  investigation  of  the  present  complex  construction 
of  the  planetary  system.  The  mind  would  have  lost  the  op- 


220  MITCHEL 

portunity  of  achieving  its  greatest  triumphs,  while  the  evi- 
dence of  infinite  wisdom  displayed  in  the  arrangement  and 
counterpoising  of  the  present  system  would  have  been  lost 
forever.  There  is  one  other  thought  which  here  suggests 
itself  with  so  much  force  that  I  can  not  turn  away  from  it. 
We  speak  of  gravitation  as  some  inherent  quality  or  prop- 
erty of  matter,  as  though  matter  could  not  exist  in  case 
it  were  deprived  of  this  quality.  This  is,  however,  a  false 
idea.  Matter  might  have  existed  independent  of  any  qual- 
ity which  should  cause  distant  globes  to  influence  each 
other.  The  force  called  gravitation,  even  admitting  that  it 
must  have  an  existence,  no  special  law  of  its  action  could 
have  forced  itself  on  matter  to  the  exclusion  of  all  other 
laws.  Why  does  this  force  diminish  as  the  square  of  the 
distance  at  which  it  operates  increases?  There  are  almost 
an  infinite  number  of  laws  according  to  which  an  attraction 
might  have  exerted  itself,  but  there  is  no  one  which  would 
have  rendered  the  planets  fit  abodes  for  sentient  beings, 
such  as  now  dwell  on  them,  and  which  would  at  the  same 
time  have  guaranteed  the  perpetuity  of  the  system.  Ad- 
mitting, then,  that  matter  can  not  be  matter  without  ex- 
erting some  influence  on  all  other  matter  (which  I  am  un- 
willing to  admit),  in  the  selection  of  the  law  of  the  inverse 
square  of  the  distance  there  is  the  strongest  evidence  of 
design. 

If  we  rise  above  the  law  of  gravitation  to  the  Great 
Author  of  Nature,  and  regard  the  laws  of  motion  and  of 
gravitation  nothing  more  than  the  uniform  expressions  of 
his  will,  we  perceive  at  once  the  impossibility  of  construct- 
ing the  universe  in  such  manner  that  the  sun  should  attract 
the  planets,  without  these  attracting  each  other;  or  that 
the  planets  should  attract  their  satellites  without,  in  turn, 
being  reciprocally  influenced  by  their  satellites;  for  this 
would  be  equivalent  to  saying  that  the  will  of  the  same 
Almighty  Being  should  exert  itself,  and  not  exert,  at  the 
same  moment,  which  is  impossible.  As  there  is  but  one 
God,  so  there  is  but  one  kind  of  matter,  governed  by  one 
law,  applied  by  infinite  wisdom  to  the  formation  of  suns 


THE   STABILITY  OF  THE   SOLAR  SYSTEM          221 

and  systems  without  number,  crowding  the  illimitable  re- 
gions of  space,  all  moving  harmoniously,  fulfilling  their 
high  destiny,  and  all  sustained  by  the  single  arm  of  divine 
Omnipotence. 

We  now  proceed  to  an  examination  of  the  great  ques- 
tion, Is  the  system  of  worlds  by  which  we  are  surrounded, 
and  of  which  our  earth  and  its  moon  form  a  part,  so  con- 
structed that,  under  the  operation  of  the  known  laws  of 
Nature,  it  shall  forever  endure,  without  ever  passing  cer- 
tain narrow  limits  of  change,  which  do  not  in  any  way 
involve  its  stability? 

It  is  well  known  that  the  planets  revolve  in  elliptical 
orbits  of  small  eccentricity — that  under  the  action  of  the 
primitive  impulse  by  which  they  were  projected  in  their 
orbits  they  would  have  moved  off  in  a  straight  line,  with 
a  velocity  proportioned  to  the  intensity  of  the  impulse,  and 
which  would  have  endured  forever;  but  being  seized  by  the 
central  attraction  of  the  sun,  at  the  moment  of  starting  in 
their  career,  the  joint  action  of  these  two  forces  bends  the 
planet  from  its  straight  direction  and  causes  it  to  commence 
a  curvilinear  path,  which  carries  it  round  the  sun. 

The  question  which  first  presents  itself  is  this:  If  the 
central  force  lodged  in  the  sun  has  the  power  to  cause  a 
planet  to  diverge  from  the  straight  line  in  which,  but  for  this, 
it  would  have  moved,  if  it  draw  it  into  a  curved  path,  will 
not  this  central  force,  which  is  ever  active,  finally  overcome 
entirely  the  impulsive  force  originally  given  to  the  planet, 
draw  it  closer  and  closer  to  the  sun  in  each  successive  revo- 
lution, in  a  spiral  orbit,  until,  finally,  the  planet  shall  fall 
into  the  sun  and  be  destroyed  forever?  This  question 
arises  independent  of  the  extraneous  influence  which  the 
planets  exert  over  each  other.  It  refers  to  a  solitary  globe 
revolving  around  the  sun,  under  the  influence  of  a  central 
force  which  varies  its  action  as  does  the  law  of  gravitation. 
The  problem  has  been  submitted  to  the  most  rigorous 
mathematical  examination,  and  a  result  has  been  obtained 
which  settles  the  question  in  the  most  absolute  manner. 
The  amount  by  which  the  central  force,  in  a  moment  of 


222  MITCHEL 

time,  overcomes  the  effect  produced  by  the  primitive  im- 
pulse, is  a  quantity  infinitely  small,  and  of  the  second  order. 
If  it  were  found  to  be  infinitely  small  in  each  moment  of 
time,  then  might  it  accumulate  so  that,  at  the  end  of  a  vast 
period,  it  might  become  finite  and  appreciable.  But  be- 
cause it  is  of  the  second  order  of  infinitely  small  quantities, 
before  it  can  become  an  infinitely  small  quantity  of  the  first 
order  a  period  equal  to  infinite  ages  must  roll  by,  and  to 
make  a  finite  appreciable  quantity  out  of  this,  an  infinite 
cycle  of  years  must  roll  round  an  infinite  number  of  times. 

Such  is  the  answer  given  by  analysis  to  this  wonderful 
question.  "  Is  there  no  change?  "  demands  the  astronomer. 
"  Yes,"  answers  the  all-seeing  analysis.  "  When  will  it  be- 
come appreciable?  "  asks  the  astronomer.  "  At  the  end  of 
a  period  infinitely  long,  repeated  an  infinite  number  of 
times/'  is  the  reply. 

Having  settled  this  important  question,  it  remains  now 
to  examine  whether  the  mutual  attractions  of  the  planets 
on  each  other  may  not,  in  the  end,  change  permanently  the 
form  of  their  orbits,  and  lead  ultimately  to  the  destruction 
of  the  system.  To  comprehend  more  readily  the  nature  of 
the  examination,  let  us  review  the  points  involved  in  the 
permanency  of  our  orbit. 

Take,  for  example,  our  own  planet,  the  earth.  It  now 
revolves  in  an  elliptic  orbit,  whose  magnitude  is  deter- 
mined by  the  length  of  its  longer  axis  and  by  its  eccen- 
tricity. These  elements  are  readily  deduced  from  observa- 
tion. If  it  were  possible  to  construct  this  orbit  of  some 
material  like  wire,  which  would  permit  us  to  take  it  up 
and  locate  it  in  space  at  will,  to  enable  us  to  give  it  the 
position  now  occupied  by  the  actual  orbit  of  the  earth,  we 
must  first  carry  its  focus  to  the  sun's  centre;  we  must  then 
turn  its  longer  axis  around  this  centre  as  a  fixed  point  until 
the  nearest  vertex  of  the  wire  orbit  shall  fall  upon  that  point 
of  the  earth's  orbit  which  is  at  this  time  nearest  to  the  sun. 
Having  accomplished  this,  the  axes  will  coincide  in  their 
entire  length,  and  to  make  the  orbits  coincide  we  must 
revolve  the  artificial  one  around  the  now  common  axis 


THE   STABILITY  OF  THE   SOLAR  SYSTEM          223 

until  its  plane  shall  fall  upon  the  actual  orbit  of  the 
earth. 

If,  now,  change  should  ever  come  in  the  absolute  coin- 
cidence of  these  two  orbits,  regarding  the  iron  one  as  fixed 
and  permanent,  the  orbit  of  Nature  may  vary  from  it  in  any 
one  or  all  of  the  following  ways:  i.  The  natural  orbit,  all 
other  things  remaining  the  same,  may  leave  the  fixed  orbit 
by  a  variation  of  eccentricity — that  is,  it  may  become  more 
or  less  nearly  circular.  2.  The  planes  of  the  orbits  remain- 
ing coincident,  the  curves  may  separate  from  each  other,  in 
consequence  of  an  angular  movement  of  the  longer  axis  of 
the  natural  orbit,  by  means  of  which  the  vertex  of  the 
natural  curve  shall  be  carried  to  the  right  or  to  the  left  of 
the  vertex  of  the  fixed  one.  3.  While  these  causes  are 
operating  to  produce  change,  an  increase  of  deviation  may 
be  occasioned  by  the  fact  that  the  two  planes  may  become 
inclined  to  each  other,  thus  causing  the  natural  orbit  to 
lie  partly  above  and  partly  below  the  fixed  one.  These, 
then,  are  the  several  ways  in  which  the  orbits  of  the  planets 
may  change;  and  to  settle  the  question  of  stability,  we 
must  ascertain  whether  these  changes  actually  exist,  and 
whether  any  of  them,  in  case  they  do  exist,  and  are  pro- 
gressing constantly  in  the  same  direction,  will  ever  prove 
fatal  to  the  permanency  of  the  system,  finally  accomplish- 
ing its  absolute  destruction,  or  rendering  it  unfit  for  the 
sustentation  of  that  life  which  now  exists  upon  the  planet. 

By  a  close  examination  of  this  great  subject,  both  the- 
oretically and  practically,  it  is  found  that  the  system  is  so 
constituted  that  not  a  single  planet  or  satellite  revolves  in 
an  orbit  absolutely  invariable.  Theory  demonstrates  that 
such  changes  must  exist,  and  observation  confirms  this 
great  truth  by  showing  that  they  actually  do  exist. 

Draw,  in  imagination,  a  straight  line  from  the  sun's 
centre  through  the  perihelion,  or  nearest  point  to  the  sun 
of  the  earth's  orbit,  and  let  it  be  extended  to  the  outermost 
limits  of  the  entire  system.  On  this  locate  the  perihelion 
points  of  the  orbits  of  all  the  planets,  and  in  these  points 
fix  the  planets  themselves.  They  are  now  all  on  the  same 


224  MITCHEL 

side  of  the  sun,  the  longer  axes  of  their  orbits  are  in  the 
same  direction,  and  they  are  located  at  their  nearest  dis- 
tance from  the  sun,  or  in  perihelion.  The  planes  of  the 
orbits  are  inclined  to  each  other  under  their  proper  angles, 
and  they  all  intersect  in  a  common  line  of  nodes  passing 
through  the  sun's  centre.  Now  give  the  entire  group  of 
planets  their  primitive  impulse,  and  at  the  same  instant  they 
start  in  their  respective  orbits  round  the  sun.  Now,  in  case 
no  perturbations  existed,  the  perihelion-points,  the  inclina- 
tions, and  the  lines  of  nodes  would  remain  fixed  forever, 
and  although  millions  of  years  might  pass  away  before  the 
planets  would  again  resume  their  primitive  position  with 
reference  to  each  other,  yet  the  time  would  come  when  a 
final  restoration  would  be  effected. 

At  the  end  of  164  years  Neptune  will  have  completed 
its  revolution  round  the  sun,  and  will  return  to  its  starting 
point.  All  the  other  planets  will  have  performed  several 
revolutions,  but  each,  on  reaching  the  point  of  departure, 
will  find  the  perihelion  of  its  orbit  changed  in  position,  the 
inclination  altered,  and  the  line  of  nodes  shifted.  These 
changes  continue  until  the  longer  axes  of  the  orbits,  which 
once  coincided,  radiate  from  the  sun  in  all  directions.  The 
lines  of  nodes,  once  common,  now  diverge  under  all  an- 
gles, the  inclinations  increasing  or  decreasing,  and  even  the 
figures  of  the  orbits  undergoing  constant  mutation;  and 
the  grand  question  arises,  whether  these  changes,  no  matter 
how  slow,  are  ever  to  continue  progressing  in  the  same 
direction  until  all  the  original  features  of  the  system  shall 
be  effaced,  and  the  possibility  of  return  to  the  primitive  con- 
dition destroyed  forever. 

Such  a  problem  would  seem  to  be  far  too  deep  and 
complicated  ever  to  be  grasped  by  the  human  intellect.  It 
is  true  that  no  single  mind  was  able  to  accomplish  its  com- 
plete solution,  but  the  advance  made  by  one  has  been 
steadily  increased  by  another,  until  finally  not  a  question 
remains  unanswered.  The  solution  is  complete,  yielding 
results  of  the  most  wonderful  character. 

We  shall  examine  this  great  problem  in  detail,  and 


THE  STABILITY  OF  THE   SOLAR  SYSTEM          225 

begin  with  the  figure  of  the  orbit  of  any  planet — our  earth, 
for  example. 

The  amount  of  heat  received  from  the  sun  by  the  earth 
depends,  other  things  being  the  same,  on  the  minor  axis 
of  its  elliptic  orbit.  Any  change  in  the  eccentricity  operates 
directly  to  increase  or  decrease  the  shorter  axis,  and  con- 
sequently to  increase  or  decrease  the  mean  annual  amount 
of  heat  received  from  the  sun.  Now,  we  know  that  animal 
and  vegetable  life  is  adjusted  in  such  a  way  that  it  re- 
quires almost  exact  uniformity  in  the  mean  annual  amount 
of  heat  which  it  shall  enjoy.  An  increase  or  decrease  of  two 
or  three  degrees  in  temperature  would  make  an  entire  revo- 
lution in  the  animals  and  plants  belonging  to  the  region 
experiencing  such  a  change.  If,  then,  it  be  true  that  the 
eccentricity  of  the  earth's  orbit  is  actually  changing  under 
the  combined  action  of  the  other  planets,  may  this  change 
continue  so  far  as  to  subvert  the  order  of  Nature  on  its 
surface?  This  question  has  been  answered  in  the  most 
satisfactory  manner. 

It  is  found  that  the  greatest  axes  of  the  planetary  orbits 
are  subjected  to  slight  and  temporary  variations,  returning 
in  comparatively  short  periods  to  their  primitive  values. 
This  important  fact  guarantees  the  permanency  of  the 
periodic  times,  so  that  it  becomes  possible  to  deduce,  with 
the  utmost  precision,  the  periodic  times  of  the  planets,  from 
the  mean  of  a  large  number  of  revolutions.  That  of  the 
earth  is  now  so  accurately  known,  and  so  absolutely  in- 
variable, that  we  know  what  it  will  be  a  million  of  years 
hence,  should  the  system  remain  as  it  now  is,  as  perfectly 
as  at  the  present  moment.  But  neither  of  these  elements 
secures  the  stability  of  the  eccentricity  or  of  the  minor  axis. 
Lagrange,  however,  demonstrated  a  relation  between  the 
masses  of  the  planets,  their  major  axes  and  eccentricities — 
such  that,  while  the  masses  remain  constant  and  the  axes 
invariable,  the  eccentricity  can  only  vary  its  value  through 
extremely  narrow  limits.  These  limits  have  been  assigned 
beyond  which  the  change  can  never  pass,  and  within  these 
narrow  bounds  we  find  that  the  orbits  of  all  the  planets 
15 


226  MITCHEL 

are  slowly  vibrating  backward  and  forward  in  periods  which 
actually  stun  the  imagination. 

This  remarkable  law  for  the  preservation  of  the  system 
would  not  hold  in  any  other  organization.  It  demands 
orbits  nearly  circular,  with  planes  nearly  coincident  with 
the  periodic  times  related  as  are  those  of  the  planets,  and 
the  planets  themselves  located  as  they  actually  are.  No 
interchange  of  orbits  is  admissible;  but,  constituted  as  the 
system  now  is,  the  perpetuity  is  absolutely  certain,  so  far 
as  the  change  of  eccentricity  is  concerned. 

Let  us  now  examine  the  changes  which  affect  the  posi- 
tion of  the  major  axis  in  its  own  plane.  The  perihelion  of 
every  orbit  is  found  to  be  slowly  advancing.  Nor  is  this 
distance  ever  to  be  changed  into  a  retrograde  motion.  The 
movement  is  ever  progressive  in  the  same  direction,  and 
the  perihelion-points  of  all  the  orbits  are  slowly  sweeping 
round  the  sun.  That  of  the  earth's  orbit  accomplishes  its 
revolution  in  111,000  years!  How  wonderful  the  fact  that 
such  discoveries  should  be  made  by  man,  whose  entire  life 
is  but  a  minute  fraction  of  these  vast  periods  of  time! 

Owing  to  a  retrograde  motion  in  the  vernal  equinox, 
carrying  it  around  in  the  opposite  direction  in  25,868  years, 
the  perihelion  and  equinox  pass  each  other  once  in  20,984 
years.  Knowing  their  relative  position  at  this  moment, 
and  their  rates  of  motion,  it  is  easy  to  compute  the  time  of 
their  coincidence.  Their  last  coincidence  took  place  4,089 
years  before  the  Christian  era,  or  about  the  epoch  usually 
assigned  for  the  creation  of  man.  The  effect  of  the  coinci- 
dence of  the  perihelion  with  the  vernal  equinox  is  to  cause 
an  exact  equality  in  the  length  of  spring  and  summer,  com- 
pared with  autumn  and  winter.  In  other  language,  the 
sun  will  occupy  exactly  half  a  year  in  passing  from  the 
vernal  to  the  autumnal  equinox,  and  the  other  half  in 
moving  from  the  autumnal  to  the  vernal  equinox. 

At  present  the  line  of  equinoxes  divides  the  earth's 
elliptic  orbit  into  two  unequal  portions.  The  smaller  part 
is  passed  over  in  the  fall  and  winter,  causing  the  earth  to 
be  nearer  the  sun  at  this  season  than  in  summer,  and  mak- 


THE  STABILITY  OF  THE  SOLAR   SYSTEM          227 

ing  a  difference  in  the  length  of  the  two  principal  seasons, 
summer  and  winter,  of  some  seventeen  and  a  half  days. 
This  inequality,  which  is  now  in  favour  of  summer,  will 
eventually  be  destroyed,  and  the  time  will  come  when  the 
earth  will  be  farthest  from  the  sun  during  the  winter  and 
nearest  in  the  summer.  But  at  the  end  of  a  great  cycle  of 
more  than  20,000  years  all  the  changes  will  have  been 
gone  through,  and  in  this  respect  a  complete  compensa- 
tion and  restoration  will  have  been  effected. 

This  epoch  of  subordinate  restoration  will  find  the  peri- 
helion of  the  earth's  orbit  located  in  space  far  distant  from 
the  point  primitively  occupied.  Five  of  these  grand  revo- 
lutions of  20,984  years  must  roll  round  before  the  slow 
movement  of  the  perihelion  shall  bring  it  back  to  its  start- 
ing point.  One  hundred  and  ten  thousand  years  will  then 
restore  the  axis  of  the  earth's  orbit,  and  the  equinoctial  line, 
nearly  to  their  relative  positions  to  each  other,  and  to  the 
same  region  of  absolute  space  occupied  at  the  beginning 
of  this  grand  cycle.  If,  now,  we  direct  our  attention  to  the 
other  planets,  we  find  their  perihelion-points  all  slowly  ad- 
vancing in  the  same  direction.  That  of  the  orbit  of  Jupiter 
performs  its  revolution  round  the  sun  in  186,207  years, 
while  the  perihelion  of  Mercury's  orbit  occupies  more  than 
200,000  years  in  completing  its  circuit  round  the  sun.  To 
effect  a  complete  restoration  of  the  planetary  orbits  to  their 
original  position  with  reference  to  their  perihelion-points 
will  require  a  grand  compound  cycle  amounting  to  millions 
of  years.  Yet  the  time  will  come  when  all  the  orbits  will 
come  again  to  their  primitive  positions,  to  start  once  more 
on  their  ceaseless  journeys. 

In  the  changes  of  the  eccentricities,  it  will  be  remem- 
bered, the  stability  of  the  system  was  involved.  Should 
these  changes  be  ever  progressive,  no  matter  how  slowly, 
a  time  would  finally  come  when  the  original  figure  of  the 
orbit  would  be  destroyed,  the  planet  either  falling  into  the 
sun,  or  sweeping  away  into  unknown  regions  of  space.  But 
a  limit  is  assigned,  beyond  which  the  change  can  never  pass. 
Some  of  the  planetary  orbits  are  becoming  more  circular, 


228  MITCHEL 

others  growing  more  elliptical;  but  all  have  their  limits 
fixed.  The  earth's  orbit,  for  example,  should  the  present 
rate  of  decrease  of  eccentricity  continue,  in  about  half  a 
million  years  will  become  an  exact  circle.  There  the  pro- 
gressive motion  of  the  changes  stops,  and  it  slowly  com- 
mences to  recover  its  ellipticity.  This  is  not  the  case  with 
the  motion  of  the  perihelions.  Their  positions  are  in  no 
way  involved  in  the  well-being  of  a  planet  or  in  its  capacity 
to  sustain  the  life  which  exists  on  its  surface;  and  since  the 
stability  of  the  system  is  not  endangered  by  progressive 
change,  it  ever  continues  in  the  same  direction,  until  the 
final  restoration  is  effected,  by  an  entire  revolution  about 
the  sun. 

Let  us  now  examine  the  inclinations  of  the  planetary 
orbits.  Here  it  is  found  that  there  is  no  guarantee  for  the 
stability  of  the  system,  provided  the  angles  under  which  the 
orbits  of  the  planets  are  inclined  to  each  other  do  not  re- 
main nearly  the  same  forever.  If  changes  are  found  to  ex- 
ist, by  which  the  inclinations  are  made  to  increase,  without 
stopping  and  returning  to  their  primitive  condition,  then 
is  the  perpetuity  of  the  system  rendered  impossible.  Its 
fair  proportions  must  slowly  wear  away,  the  harmony  which 
now  prevails  be  destroyed,  and  chaos  must  come  again. 

Commencing  again  with  the  earth,  we  find  that  from 
the  earliest  ages  the  inclination  of  the  earth's  equator  to 
the  ecliptic  has  been  decreasing.  Since  the  measure  of 
Eratosthenes,  2,078  years  ago,  the  decrease  has  amounted 
to  about  23'  44",  or  about  half  a  second  every  year. 
Should  the  decrease  continue,  in  about  85,000  years  the 
equator  and  ecliptic  would  coincide,  and  the  order  of  Na- 
ture would  be  entirely  changed;  perpetual  spring  would 
reign  throughout  the  year,  and  the  seasons  would  be  lost 
forever.  Of  this,  however,  there  is  no  danger.  The  diminu- 
tion will  reach  its  limit  in  a  comparatively  short  time,  when 
the  decrease  of  inclination  will  change  into  an  increase,  and 
thus  slowly  rocking  backward  and  forward  in  thousands  of 
years,  the  seasons  shall  ever  preserve  their  appointed  places, 
and  seedtime  and  harvest  shall  never  fail.  These  changes 


THE   STABILITY   OF  THE   SOLAR   SYSTEM          229 

of  inclination  are  principally  due  to  the  perturbations  of 
Venus,  and,  arising  from  configurations,  will  be  ultimately 
entirely  compensated. 

The  angles  under  which  the  planetary  orbits  are  in- 
clined to  each  other  are  in  a  constant  state  of  mutation. 
The  orbit  of  Jupiter  at  this  time  forms  an  angle  with  the 
ecliptic  of  4,731  seconds,  and  this  angle  is  decreasing  at 
such  a  rate  that  in  about  20,000  years  the  planes  would 
actually  coincide.  This  would  not  affect  the  well-being  of 
the  planets  or  the  stability  of  the  system;  but  should  the 
same  change  now  continue,  the  angle  between  the  orbits 
might  finally  come  to  fix  them  even  at  right  angles  to  each 
other,  and  a  subversion  of  the  present  system  would  result. 

A  profound  investigation  of  the  problem  of  the  plan- 
etary inclinations,  accomplished  by  Lagrange,  resulted  in 
the  demonstration  of  a  relation  between  the  masses  of  the 
planets,  the  principal  axes  of  their  orbits,  and  the  inclina- 
tions, such  that,  although  the  angles  of  inclination  may 
vary,  the  limits  are  narrow,  and  they  are  all  found  slowly 
to  oscillate  about  their  mean  positions,  never  passing  the 
prescribed  limits,  and  securing  in  this  particular  the  per- 
petuity of  the  system. 

Here,  again,  we  are  presented  with  the  remarkable  fact 
that  whenever  mutation  involves  stability,  this  mutation  is 
of  a  compensatory  character,  always  returning  upon  itself, 
and  in  the  long  run  correcting  its  own  effects.  If  all  this 
mighty  system  was  organized  by  chance,  how  happens  it 
that  the  angular  motions  of  the  perihelia  of  the  planetary 
orbits  are  ever  progressive,  while  the  angular  motions  of 
the  planes  of  the  orbits  are  vibrating?  Design,  positive  and 
conspicuous,  is  written  all  over  the  system  in  characters 
from  which  there  is  no  escape. 

We  now  proceed  to  an  examination  of  the  lines  in 
which  the  planes  of  the  planetary  orbits  cut  each  other,  or 
the  lines  in  which  they  intersect  a  fixed  plane.  These  are 
called  the  lines  of  nodes.  They  all  pass  through  the  sun's 
centre,  and,  in  case  they  ever  were  coincident,  they  now 
radiate  from  a  common  point  in  all  directions. 


230 


MITCHEL 


Here  is  an  element  in  no  degree  involving  in  its  value 
the  stability  of  the  system,  and  from  analogy  we  already 
begin  to  anticipate  that  its  changes,  whatever  they  may  be, 
will  probably  progress  always  in  the  same  direction.  This 
is  actually  the  case.  The  nodes  of  the  planetary  orbits  are 
all  slowly  retrograding  on  a  fixed  plane;  and  in  vast  periods, 
amounting  to  thousands  of  years,  accomplish  revolutions 
which  in  the  end  return  them  to  their  primitive  positions. 

Thus  are  we  led  to  the  following  results:  Of  the  two 
elements  which  fix  the  magnitude  of  the  planetary  orbits, 
the  principal  axes,  and  the  eccentricity,  the  axes  remain 
invariable,  while  the  eccentricity  oscillates  between  narrow 
and  fixed  points.  In  the  Jong  run,  therefore,  the  magni- 
tudes of  the  orbits  are  preserved. 

Of  the  three  elements  which  give  position  to  the  plan- 
etary orbits — viz.,  the  place  of  the  perihelion,  the  lines  of 
nodes,  and  the  inclinations — the  first  two  ever  vary  in  the 
same  direction,  and  accomplish  their  restoration  at  the 
end  of  vast  periods  of  revolution,  while  the  inclinations 
vibrate  between  narrow  and  prescribed  limits. 

One  more  point,  and  we  close  this  wonderful  investiga- 
tion. The  last  question  which  presents  itself  is  this:  May 
not  the  periodic  times  of  the  planets  be  so  adjusted  to  each 
that  the  results  of  certain  configurations  may  ever  be 
repeated  without  any  compensation,  and  thus,  by  perpetual 
accumulation,  finally  effect  a  destruction  of  the  system? 

If  the  periodic  times  of  two  neighbourng  planets  were 
exact  multiples  of  the  same  quantity,  or  if  the  one  was 
double  the  other,  or  in  any  exact  ratio,  then  the  contin- 
gency above  alluded  to  would  arise,  and  there  would  be 
perturbations  which  would  remain  uncompensated.  A  near 
approach  to  this  condition  of  things  actually  exists  in  the 
system,  and  gave  great  trouble  to  geometers.  It  was  found, 
in  comparing  observations,  that  the  mean  periods  of  Jupiter 
and  Saturn  were  not  constant — that  one  was  on  the  de- 
crease while  the  other  was  on  fhe  increase.  This  discovery 
seemed  to  disprove  the  great  demonstration  which  had 
fixed  as  invariable  the  major  axes  of  the  planetary  orbits, 


THE   STABILITY  OF  THE   SOLAR  SYSTEM          231 

and  guaranteed  the  stability  of  the  mean  motions.  It  was 
not  until  after  Laplace  had  instituted  a  long  and  laborious 
research  that  the  phenomenon  was  traced  to  its  true  origin, 
and  was  found  to  arise  from  the  near  commensurability  of 
the  periodic  times  of  Jupiter  and  Saturn — five  of  Jupiter's 
periods  being  nearly  equal  to  two  of  Saturn's.  In  case 
the  equality  were  exact,  it  is  plain  that  if  the  two  planets 
set  out  from  the  same  straight  line  drawn  from  the  sun, 
at  the  end  of  a  cycle  of  five  of  Jupiter's  periods,  or  two  of 
Saturn's,  they  would  be  again  found  in  the  same  relative 
positions,  and  whatever  effect  the  one  planet  had  exerted 
over  the  other  would  again  i>e  repeated  under  the  same 
precise  circumstances.  Hence  would  arise  derangements 
which  would  progress  in  the  same  direction,  and  eventually 
lead  to  the  permanent  derangement  of  the  system. 

But  it  happens  that  five  of  Jupiter's  periods  are  not  ex- 
actly equal  to  two  of  Saturn's,  and  in  this  want  of  equality 
safety  is  found.  The  difference  is  such  that  the  point  of 
conjunction  of  the  planets  does  not  fall  at  the  same  points 
of  their  orbits,  but  at  the  end  of  each  cycle  is  in  advance 
by  a  few  degrees.  Thus  the  conjunction  slowly  works 
round  the  orbits  of  the  planets,  and  in  the  end  the  effect 
produced  on  one  side  of  the  orbit  is  compensated  for  on 
the  other,  and  a  mean  period  of  revolution  comes  out  for 
both  planets,  which  is  invariable.  In  the  case  of  Jupiter 
and  Saturn,  the  entire  compensation  is  not  effected  until 
after  a  period  of  nearly  a  thousand  years. 

A  similar  inequality  is  found  to  exist  between  the  earth 
and  Venus,  with  a  period  much  shorter,  and  producing  re- 
sults much  less  easily  observed.  In  no  instance  do  we  find 
the  periods  of  any  two  planets  in  an  exact  ratio.  They  are 
all  incommensurable  with  each  other,  and  in  this  peculiar 
arrangement  we  find  the  stability  of  the  entire  system  is 
secured. 

So  far,  then,  as  the  organization  of  the  great  planetary 
system  is  concerned,  we  do  not  find  within  itself  the  ele- 
ments of  its  own  destruction.  Mutation  and  change  are 
everywhere  found — all  is  in  motion,  orbits  expanding  or 


232  MITCHEL 

contracting,  their  planes  rocking  up  and  down,  their  peri- 
helia and  nodes  sweeping  in  opposite  directions  round  the 
sun — but  the  limits  of  all  these  changes  are  fixed;  these 
limits  can  never  be  passed,  and  at  the  end  of  a  vast  period, 
amounting  to  many  millions  of  years,  the  entire  range  of 
fluctuation  will  have  been  accomplished,  the  entire  system 
— planets,  orbits,  inclinations,  eccentricities,  perihelia,  and 
nodes — will  have  regained  their  original  values  and  places, 
and  the  great  bell  of  eternity  will  have  then  sounded  "  One." 

Having  reached  the  grand  conclusion  of  the  stability  of 
the  system  of  planets  in  their  reciprocal  influences,  and 
that  no  element  of  destruction  is  found  in  the  organization, 
we  propose  next  to  inquire  whether  the  same  features  are 
stamped  on  the  subordinate  groups  composing  the  plan- 
etary system.  As  our  limits  will  not  permit  us  to  enter  into 
a  full  examination  of  all  the  subordinate  groups,  we  shall 
confine  our  remarks  to  our  own  earth  and  its  satellite, 
Jupiter  and  his  satellites,  and  to  Saturn,  his  rings  and 
moons.  We  shall,  in  this  examination,  find  it  practicable 
to  answer,  to  some  extent,  the  inquiry  as  to  whether  either 
of  these  systems  has  received  any  shock  from  external 
causes.  We  know  nothing  as  to  the  future,  and  can,  in  this 
particular,  only  form  our  conjectures  as  to  what  is  to  be 
from  what  has  been. 

We  commence  our  inquiry  by  an  examination  of  two 
questions — viz.,  Is  the  velocity  of  rotation  of  the  earth  on 
its  axis  absolutely  invariable?  Has  the  relation  between 
the  earth  and  moon  ever  been  disturbed  by  any  external 
cause?  There  is  nothing  so  important  to  the  well-being 
of  our  planet  and  its  inhabitants  as  absolute  invariability  in 
the  period  of  its  axial  rotation.  The  sidereal  day  is  the 
great  unit  of  measure  for  time,  and  is  of  the  highest  conse- 
quence in  all  astronomical  investigations.  If  causes  are 
operating,  either  to  increase  or  decrease  the  velocity  of 
rotation,  a  time  will  come  when  the  earth  will  cease  to 
rotate,  or  else  acquire  so  great  a  velocity  as  to  destroy  its 
figure,  and,  in  the  end,  scatter  its  particles  in  space. 

It  is  difficult  to  ascertain  from  theory  a  perfectly  satis- 


THE  STABILITY  OF  THE   SOLAR  SYSTEM          233 

factory  answer  to  the  question  of  the  invariable  velocity  of 
rotation  of  the  earth,  but  Laplace  has  demonstrated  that 
the  length  of  the  day  has  not  varied  by  the  hundredth  of 
one  second  during  the  last  2,000  years — that  is,  the  length 
of  the  day  is  neither  greater  nor  less  than  it  was  2,000  years 
ago  by  the  hundredth  of  a  second.  The  reasoning  leading 
to  this  remarkable  result  is  simple,  and  may  be  readily  com- 
prehended by  all.  Two  thousand  years  ago  the  duration 
of  the  moon's  period  of  revolution  around  the  earth  was 
accurately  determined  and  was  expressed  in  days  and  parts 
of  a  day.  The  measure  of  the  same  period  has  been  ac- 
complished in  our  own  time,  and  is  expressed  in  days  and 
parts  of  a  day.  Now,  all  the  causes  operating  to  change  the 
moon's  period  of  revolution  are  known,  and  may  be  applied. 
When  this  is  done,  it  is  found  that  the  moon's  period  now 
and  2,000  years  ago  agree  precisely,  being  accomplished 
in  the  same  number  of  days  and  parts  of  a  day — which 
would  be  impossible  if  the  unit  of  a  measure,  the  day,  had 
varied  ever  so  slightly. 

The  extraordinary  relation  existing  between  the  moon's 
period  in  her  orbit  and  the  time  occupied  in  her  axial  rota- 
tion gives  us  the  opportunity  of  ascertaining  whether  our 
system  has  received  any  external  shock.  These  two  periods 
are  so  accurately  adjusted  that  in  all  respects  an  exact 
equality  exists.  The  moon  ever  turns  the  same  hemisphere 
to  the  earth,  and  ever  will,  unless  some  external  cause 
should  arise  to  disturb  the  perfect  harmony  which  now 
reigns.  It  is  not  my  purpose  to  explain  why  it  is  that  this 
phenomenon  exists.  I  merely  desire  to  state  that  this  deli- 
cate balancing  of  periods  furnishes  an  admirable  evidence 
that,  for  several  thousand  years,  at  least  no  shock  has  been 
received  by  the  earth  and  its  satellite.  Steadily  have  they 
moved  in  their  orbits,  subject  only  to  the  influence  of  causes 
originating  in  the  constitution  of  the  mighty  system  of 
which  they  constitute  a  part. 

Moving  out  to  a  more  complex  system,  we  find  in  the 
remarkable  arrangement  of  the  satellites  of  Jupiter  a  delicate 
test  for  the  action  of  sudden  and  extraneous  causes.  Here 


234 


MITCHEL 


we  find  the  periodic  times  of  the  satellites  so  related  that 
1,000  periods  of  the  first  added  to  2,000  periods  of  the  third 
will  be  precisely  equal  to  3,000  periods  of  the  second.  This 
delicate  balancing  of  periods  would  be  destroyed  by  the  ac- 
tion of  any  external  shock,  such  as  might  be  experienced 
from  the  collision  of  a  comet  sweeping  through  the  system. 
Thus  far  we  know  that  no  disturbance  has  entered,  and  a 
knowledge  of  facts  will  now  pass  down  to  posterity,  which 
will  give  the  means  of  ascertaining  exactly  the  influence  of 
all  disturbing  causes  which  do  not  form  a  part  of  the  great 
system. 

The  last  subordinate  group,  and  the  most  extraordinary 
one  to  which  I  will  at  this  time  direct  your  attention,  is 
that  of  Saturn  and  his  rings.  Here  we  find  a  delicacy  of  ad- 
justment and  equilibrium  far  exceeding  anything  yet  ex- 
hibited in  our  examinations.  This  great  planet  is  sur- 
rounded certainly  by  two,  probably  by  three,  immense 
rings,  which  are  formed  of  solid  matter,  in  all  respects  like 
that  constituting  the  central  body.  These  wonderful  ap- 
pendages are  nowhere  else  to  be  found  throughout  the 
entire  solar  system,  at  least  with  certainty.  Their  existence 
has  elsewhere  been  suspected,  but  around  Saturn  they  are 
seen  with  a  perfection  and  distinctness  which  defies  all 
scepticism  as  to  their  actual  existence.  The  diameter  of 
the  outer  ring  is  no  less  than  176,000  miles.  Its  breadth  is 
21,000  miles,  while  its  thickness  does  not  exceed  100  miles. 
The  inner  ring  is  separated  from  the  outer  one  by  a  space 
of  about  1, 800  miles,  its  breadth  34,000  miles,  its  inner  edge 
being  about  20,000  miles  from  the  surface  of  the  planet. 
Its  thickness  is  the  same  as  that  of  the  outer  ring.  These 
extraordinary  objects  are  rotating  in  the  same  direction 
as  the  planet,  and  with  a  velocity  so  great  that  objects  on 
the  extreme  edge  of  the  outer  ring  are  carried  through 
space  with  the  amazing  velocity  of  nearly  50,000  miles  an 
hour,  or  nearly  fifty  times  more  swiftly  than  the  objects  on 
the  earth's  equator. 

What  power  of  adjustment  can  secure  the  stability  of 
these  stupendous  rings?  No  solid  bond  fastens  them  to 


THE   STABILITY  OF  THE   SOLAR  SYSTEM          235 

the  planet;  isolated  in  space,  they  hold  their  places,  and 
revolving  with  incredible  velocity  around  an  imaginary 
axis,  they  accompany  their  planet  in  its  mighty  orbit  round 
the  sun.  Such  is  the  exceeding  delicacy  with  which  this 
system  is  adjusted,  that,  the  slightest  external  cause  once 
deranging  the  equilibrium,  no  readjustment  would  be 
effected.  The  rings  would  be  thrown  on  the  body  of  the 
planet,  and  the  system  would  be  destroyed. 

To  understand  the  extraordinary  character  of  this  sys- 
tem we  will  explain  a  little  more  fully  the  three  different 
kinds  of  equilibrium.  The  first  is  called  an  equilibrium  of 
instability,  and  is  exemplified  in  the  effort  to  balance  a  rod 
on  the  tip  of  the  finger.  The  slightest  deviation  from  the 
exact  vertical  increases  itself  constantly,  until  the  equilib- 
rium is  destroyed.  In  case  the  same  rod  be  balanced  on 
its  centre  on  the  finger,  it  presents  an  example  of  an  equi- 
librium of  indifference — that  is,  if  it  be  swayed  slightly  to 
the  one  side  or  the  other,  there  is  no  tendency  to  restore 
itself,  or  to  increase  its  deviation.  It  remains  indifferent  to 
any  change.  Take  the  same  rod,  and  suspend  it  like  a  pen- 
dulum. Now  cause  it  to  deviate  from  the  vertical  to  the 
right  or  left,  and  it  returns  of  itself  to  the  condition  of 
equilibrium.  This  is  an  equilibrium  of  stability.  We  have 
already  seen  that  this  is  the  kind  of  equilibrium  which  exists 
in  the  planetary  system.  There  are  constant  deviations, 
but  a  perpetual  effort  is  making  to  restore  the  object  to  its 
primitive  condition. 

Now,  in  case  the  rings  of  Saturn  are  homogeneous, 
equally  thick,  and  exactly  concentric  with  the  planet,  their 
equilibrium  is  one  of  instability.  The  smallest  derange- 
ment would  find  no  restorative  power,  and  would  even  per- 
petuate and  increase  itself,  until  the  system  is  destroyed. 
For  a  long  time  it  was  believed  that  the  rings  were  equally 
thick,  and  concentric  with  the  planet,  but  when  it  was  dis- 
covered that  such  features  would  produce  an  equilibrium  of 
instability,  and  that  there  existed  no  guarantee  for  the  per- 
manency of  this  exquisite  system,  an  analytic  examination 
was  made,  which  led  to  this  singular  result — viz.,  to  change 


236       ,  *  MITCHEL 

the  equilibrium  of  instability  into  one  of  stability,  all  that 
is  necessary  is  to  make  the  ring  thicker  or  denser  in  some 
parts  than  in  others,  and  to  cause  its  centre  of  position  to 
be  without  the  centre  of  the  planet,  and  to  perform  around 
that  centre  a  revolution  in  a  minute  orbit.  Finding  these 
conditions  analytically,  it  now  became  a  matter  of  deep 
interest  to  ascertain  whether  these  conditions  actually  ex- 
isted in  Nature.  The  occasional  disappearance  of  the  ring, 
in  consequence  of  its  edge  being  presented  to  the  eye  of 
the  observer,  gave  a  capital  opportunity  of  determining 
whether  it  was  of  uniform  thickness.  On  these  rare  occa- 
sions, in  the  most  powerful  telescopes,  the  ring  remains 
visible  edgewise,  and  looks  like  a  slender  fibre  of  silver  light 
drawn  across  the  diameter  of  the  planet.  In  the  gradual 
wasting  away  of  the  two  extremities  of  the  ring  it  has  been 
remarked  that  the  one  remains  visible  longer  than  the  other. 
As  the  ring  is  swiftly  revolving,  neither  extremity  can,  in 
any  sense,  be  regarded  as  fixed,  and  hence  sometimes  the 
one,  sometimes  the  other,  fades  first  from  the  sight.  An 
exactly  uniform  thickness  in  the  ring  would  render  such  a 
phenomenon  impossible,  and  hence  we  conclude  that  the 
first  condition  of  stability  is  fulfilled — the  rings  are  not 
equally  thick  throughout. 

The  micrometer  was  now  applied  to  detect  an  eccen- 
tricity in  the  central  point  of  the  ring.  Recent  examina- 
tions by  Struve  and  Bessel  have  settled  this  question  in 
the  most  satisfactory  manner.  The  centre  of  the  ring  does 
not  coincide  with  that  of  the  planet,  and  it  is  actually  per- 
forming a  revolution  around  the  centre  of  the  planet  in 
a  minute  orbit,  thus  forming  the  second  delicate  condition 
of  equilibrium.  The  analogy  of  the  great  system  is  un- 
broken in  the  subordinate  one.  For  more  than  two  hundred 
years  have  these  wonderful  circles  of  light  whirled  in  their 
rapid  career  under  the  eye  of  man,  and  freed  from  all  ex- 
ternal action  they  are  so  poised  that  millions  of  years  shall 
in  nowise  affect  their  beautiful  organization.  Their  graceful 
figures  and  beautiful  light  shall  greet  the  eye  of  the  student 
when  ten  thousand  years  shall  have  rolled  away. 


THE   STABILITY  OF  THE   SOLAR   SYSTEM          237 

Thus  do  we  find  that  God  has  built  the  heavens  in  wis- 
dom, to  declare  his  glory,  and  to  show  forth  his  handi- 
work. There  are  no  iron  tracks,  with  bars  and  bolts,  to 
hold  the  planets  in  their  orbits.  Freely  in  space  they 
move,  ever  changing,  but  never  changed;  poised  and  bal- 
ancing; swaying  and  swayed;  disturbing  and  disturbed, 
onward  they  fly,  fulfilling  with  unerring  certainty  their 
mighty  cycles.  The  entire  system  forms  one  grand  com- 
plicated piece  of  celestial  machinery — circle  within  circle, 
wheel  within  wheel,  cycle  within  cycle — revolution  so  swift 
as  to  be  completed  in  a  few  hours;  movements  so  slow  that 
their  mighty  periods  are  only  counted  by  millions  of  years. 
Are  we  to  believe  that  the  Divine  Architect  constructed 
this  admirably  adjusted  system  to  wear  out,  and  to  fall 
in  ruins,  even  before  one  single  revolution  of  its  complex 
scheme  of  wheels  had  been  performed?  No.  I  see  the 
mighty  orbits  of  the  planets  slowly  rocking  to  and  fro,  their 
figures  expanding  and  contracting,  their  axes  revolving  in 
their  vast  periods;  but  stability  is  there.  Every  change 
shall  wear  away,  and  after  sweeping  through  the  grand 
cycle  of  cycles,  the  whole  system  shall  return  to  its  primitive 
condition  of  perfection  and  beauty. 


THE  NEW  PLANET,  EROS 


BY 

EDMUND   LEDGER 


THE   NEW    PLANET,   EROS1 

ON  the  1 3th  of  August,  1898,  a  little  planet  was  dis- 
covered at  the  Urania  Observatory  of  Berlin  by 
Herr  G.  Witt,  to  which  he  has  since  given  the  name 
of  Eros.  Its  discovery  has  been  the  great  astronomical 
sensation  of  the  past  twelve  months,  because  its  orbit  passes 
out  of  the  zone  in  which  all  the  other  small  planets  move 
into  a  very  remarkable  proximity  to  the  earth — a  proximity 
which  will  cause  Eros  to  be  of  the  highest  value  in  con- 
nection with  some  of  the  most  important  problems  of 
astronomy. 

In  order  to  appreciate  the  method  of  its  discovery,  and 
the  reasons  which  make  that  discovery  so  important,  it  will 
be  well  briefly  to  recall  what  had  been  previously  achieved 
in  the  same  line  of  research.  Copernicus  had  proved  and 
Kepler  and  others  had  drawn  attention  to  the  greatness  of 
the  gap  between  the  orbits  of  Mars  and  Jupiter.  Then,  in 
1781,  Sir  William  Herschel  detected  the  planet  Uranus  at 
a  distance  from  the  sun  agreeing  with  the  next  term  of  a 
series  which  Titius  and  Bode  had  noticed  as  almost  exactly 
representing  the  distances  of  the  other  planets,  except  that 
for  one  term,  between  those  which  corresponded  with  Mars 
and  Jupiter,  there  was  no  planet  known. 

While  twenty-four  astronomers  were  arranging  a  search 
for  such  a  missing  member  of  the  solar  system,  Piazzi  (who 
was  not  one  of  them)  unexpectedly  detected  at  Palermo, 
on  the  ist  of  January,  1801,  a  little  planet,  afterward  named 
Ceres.  Three  more  were  found  by  two  out  of  the  twenty- 
four  astronomers  in  the  course  of  the  next  six  years.  A 

1  From  the  "  Nineteenth  Century,"  April,  1899. 
16  241 


242  LEDGER 

fifth  was  found  in  1845,  after  an  interval  of  thirty-eight 
years;  then  the  progress  became  rapid.  Since  1846  no 
year  has  passed  without  such  a  discovery.  In  1868  the 
total  reached  100;  in  1879,  200;  in  1890,  300;  in  1895, 
400;  and  now  it  is  nearly  450.  After  a  few  of  the  brighter 
ones  had  been  detected  the  search  for  others  became  a 
wearisome  process.  New  star  charts  had  to  be  constructed, 
with  great  labour,  so  as  to  include  the  fainter  stars.  If  a 
small  star  was  noticed  with  the  telescope  which  could  not 
be  found  in  them,  it  was  carefully  watched,  and  if  it  ex- 
hibited an  orbital  movement  it  was  entered  on  the  list  of 
planets,  and  the  various  elements  of  its  orbit  were  calculated 
and  recorded. 

But  when  many  astronomers  were  inclined  to  look  upon 
all  this  work  as  well-nigh  profitless,  and  too  wearisome  to 
be  continued,  photography  with  startling  suddenness  did 
away  with  all  need  of  star  charts  and  of  any  comparison  of 
observations  with  them.  Upon  a  photographic  plate  suit- 
ably exposed,  Herr  Max  Wolf,  of  Heidelberg,  found  that 
a  little  planet  had  recorded  its  place  on  the  226.  of  Decem- 
ber, 1891.  There  was  no  need  to  compare  the  plate  with 
any  chart  of  stars.  The  planet  had  asserted  its  right  to  the 
name  of  Wanderer  by  moving  on  a  little  way  in  its  orbit 
among  the  stars  during  the  exposure  of  the  plate.  The  stars 
left  their  traces  in  dots  (the  effect  of  the  rotatory  motion 
of  the  earth  having  been  duly  compensated);  the  planet 
left  its  trail  in  a  little  straight  line  drawn  by  it  in  the  direc- 
tion of  its  motion. 

In  the  next  year  Max  Wolf  found  thirteen  more,  and 
in  the  same  year  Charlois  ten,  by  this  new  method.  And 
since  the  early  part  of  1892,  out  of  one  hundred  discoveries 
of  such  planets,  only  three  or  four  have  been  made  by  the 
old  method  of  eye  observation.  Once  more,  however,  the 
great  abundance  of  these  photographic  discoveries  of  planet 
after  planet  began  to  make  astronomers  despair  of  the  pos- 
sibility of  so  keeping  count  of  their  orbits  and  positions  as 
to  be  able  to  determine  whether  the  little  trails,  of  which 
several  were  sometimes  found  upon  the  same  photographic 


THE   NEW  PLANET.  EROS  243 

plate,  indicated  the  presence  of  planets  previously  seen  or 
hitherto  unknown.  It  would,  indeed,  have  been  quite  im- 
possible to  do  so  had  it  not  been  for  the  unremitting  in- 
dustry of  German  computers,  as  evidenced  year  by  year  in 
the  "  Berliner  Jahrbuch." 

These  little  bodies  were  even  termed  astronomical  nui- 
sances. But  one  of  them — the  433d — has  at  last  proved 
to  be  a  great  astronomical  treasure.  It  has  proved  that 
it  would  have  been  most  unwise  to  have  neglected  any  of 
these  minute  portions  of  our  solar  system.  Some — e.  g., 
Hilda  (No.  153),  Thule  (No.  279),  and  one  which  is  still 
unnamed  (No.  361) — approach  so  near  to  the  orbit  of 
Jupiter  that  they  will  be  of  much  use  in  the  accurate  de- 
termination of  that  great  planet's  mass.  Others  are  of 
especial  interest  in  the  comparison  of  their  very  oval  orbits 
with  those  of  certain  periodic  comets.  But  by  far  the  most 
important  are  those  whose  orbits  lie  nearest  to  that  of  the 
earth.  Only  three  or  four,  however,  such  as  JEthra  (No. 
132),  Brucia  (No.  323),  and  Ingeborg  (No.  391),  have 
hitherto  been  found  which  approach  the  earth,  even  to  a 
very  moderate  extent,  within  the  distance  of  that  part  of 
the  somewhat  oval  orbit  of  Mars  in  which  he  is  at  his  far- 
thest from  the  sun;  and  they  do  so  only  in  a  small  portion 
of  their  orbits. 

But  in  the  case  of  Eros  we  meet  with  something  utterly 
different  and  unexpected.  A  new  planet  has  been  dis- 
covered whose  average  distance  from  the  sun  is  less  than 
that  of  Mars;  a  planet  which  at  times  comes  within  a  dis- 
tance from  the  earth  not  much  more  than  one  third  of  the 
nearest  distance  within  which  Mars  ever  approaches  it. 

On  the  1 3th  of  August,  1898,  Herr  Witt  exposed  a 
photographic  plate  with  the  hope  of  obtaining  upon  it  the 
trail  of  another  previously  known  minor  planet.  He  suc- 
ceeded, but  upon  the  plate  there  was  also  a  second,  fainter 
trail — faint  and  of  unusual  length  because  of  the  rapidity 
with  which  the  planet  had  moved.  This  indicated  an  un- 
usual orbit.  Further  observations  were  at  once  made. 
From  them  Herr  Berberich  calculated  what  proved  to  be 


244  LEDGER 

a  most  surprising  orbit.  The  path  of  Mars  up  to  this  time 
had  practically  formed  the  boundary  beyond  which  minor 
planets  had  hardly  transgressed.  This  new  planet  came 
45,000,000  miles  within  the  mean  distance  of  Mars.  With 
the  exception  of  the  moon  it  is  by  far  the  nearest  celestial 
neighbour  of  the  earth,  the  nearest  approach  even  of  Venus 
to  the  earth  being  not  much  less  than  twice  as  great. 

But  let  us  now  ask,  Why  should  the  near  approach  of 
Eros  to  the  earth  attach  an  extraordinary  value  to  our  ac- 
quaintance with  it?  Is  it  because  we  may  hope  to  see  the 
details  of  its  surface,  or  to  set  up  some  communication  be- 
tween it  and  the  earth?  By  no  means.  If  we  may  judge 
by  the  amount  of  light  which  it  reflects  to  us,  we  may  con- 
clude that  its  diameter  is  probably  less  than  twenty  miles. 
The  largest  telescope,  therefore,  will  barely  reveal  in  it 
any  disk  of  measurable  breadth.  On  the  contrary,  the  great 
value  of  this  little  Eros  depends  upon  its  enabling  us  to 
measure  the  scale  upon  which  the  whole  universe  around 
us  is  constructed  with  an  accuracy  much  surpassing  any  that 
has  been  previously  attained. 

Our  estimate,  for  instance,  of  the  distance  of  any  star, 
or  of  the  size  of  the  orbits  of  any  pair  of  double  stars,  in 
fact,  all  our  measurements  in  the  celestial  spaces,  depend 
upon  our  knowledge  of  the  distance  of  the  earth  from  the 
sun.  To  determine  that  distance  a  direct  trigonometrical 
method,  such  as  is  used  in  surveying,  and  such  as  may  be 
applied  to  find  the  distance  of  the  moon  from  the  earth, 
can  not  be  used,  as  no  instruments  can  be  constructed  of 
the  necessary  delicacy.  But  there  is  a  remarkable  propor- 
tion connected  with  the  movements  of  the  planets  in  their 
orbits,  discovered  by  Kepler  and  more  fully  investigated  by 
the  genius  of  Newton,  which  enables  us  at  once  to  deter- 
mine the  distance  of  the  sun,  if  only  we  can  measure  the 
distance  of  any  one  of  the  other  planets  from  the  earth. 

It  was  at  one  time  hoped  that  this  might  be  accurately 
determined  in  the  case  of  Venus  by  observations  made  on 
those  rare  occasions  when  it  passes  in  transit  across  the 
sun's  disk.  But  the  glare  of  the  sun's  light,  the  ill-defined 


FIRST  OBSERVATION  OF  THE   TRANSIT 
OF  VENUS. 

Photogravure  from  an  old  etching. 


a  most  surprisii:  ,ath  of  Mars  up  to  this  time 

had  practically  i  adary  beyond  which  minor 

P!anc  This  new  planet  came 

45,000,0*  m  distance  of  Mars.    With 

it  is  by  far  the  nearest  celestial 

Approach  even  of  Venus 

han  twice  as  great. 

Id  the  near  approach  of 

ary  value  to  our  ac- 

fiope  to  see  the 

imunication  be- 

may  judge 

may  con- 


ie^rhole  universe  around 
that 


nee  of  any  star, 
•s,  in 
uretnent^ 


>f  the  d  ,,i  the 

listance  a  <••  umetrical 

i  the  earth, 

istructed  of 

narkable  propor- 

planets  in  their 

investigated  by 

us  at  once  to  deter- 

we  can  measure  the 

clistr>  ets  from  the  earth. 

iiis  might  be  accunr 

^rmined  ^  by  observations  made 

•;e  rare  ?>en  it  passes  in  transit  across 

isk.    But  the  glare  of  the  sun's  light,  the  ill-' 


THE  NEW  PLANET,    EROS  245 

edge  of  the  sun's  disk,  and  the  atmosphere  of  Venus  itself, 
combine  to  deprive  such  observations  of  the  necessary  ac- 
curacy. Apart  from  some  other  methods,  involving  long 
periods  of  time  and  highly  complicated  theoretical  investi- 
gations in  their  use,  attention  was  therefore  next  given  to 
an  attempt  to  obtain  the  distance  of  the  planet  Mars  when 
it  makes  its  nearest  approaches  to  the  earth.  It  was,  how- 
ever, found  to  be  difficult  to  measure  the  exact  position  of 
the  centre  of  its  disk.  Whereupon  it  was  suggested  that 
some  of  the  nearer  minor  planets,  although  they  would  be 
farther  from  the  earth,  and  their  distance  from  it  propor- 
tionately more  difficult  to  determine,  might  more  than  com- 
pensate for  this  disadvantage  by  the  great  accuracy  with 
which  the  positions  of  their  starlike  telescopic  images  might 
be  observed.  And  this  was  found  to  be  the  case.  The 
most  accurate  value  of  the  sun's  distance  known  at  the 
present  time  is  believed  to  be  that  which  has  been  skilfully 
deduced  in  this  way  from  observations  of  certain  of  the 
nearer  minor  planets  by  Dr.  Gill,  H.  M.  Astronomer  at  the 
Cape  of  Good  Hope. 

The  new  planet,  Eros,  is  of  the  utmost  value  for  such 
observations,  because  the  accuracy  of  the  result  which  they 
afford  is  proportionate  to  the  nearness  to  the  earth  of  the 
planet  that  is  observed.  The  method  of  calculation  em- 
ployed depends  upon  the  ratio  of  the  planet's  distance  to 
the  distance  between  two  observers  simultaneously  look- 
ing at  it  from  two  widely  separated  points  upon  the 
earth's  surface,  or  to  the  distance  through  which  an  ob- 
server may  himself  be  moved,  by  the  rotation  of  the  earth 
upon  its  axis,  between  two  observations  made,  the  one 
soon  after  sunset,  and  the  other  shortly  before  sunrise.  The 
movements  of  the  earth  and  the  planet  in  their  orbits  in  the 
interval  (as  also  if  in  the  first  case  the  two  observations 
made  are  not  exactly  simultaneous)  can  be  allowed  for,  and 
will  not  affect  the  final  result.  An  observer  may  be  moved 
between  such  an  evening  and  early-morning  observation 
when  this  latter  (termed  the  diurnal)  method  is  employed, 
provided  he  be  near  to  the  equator,  to  a  position  which  may 


246  LEDGER 

be  separated  by  about  7,000  miles,  supposed  to  be  measured 
in  a  straight  line  drawn  through  the  earth,  from  his  pre- 
vious place.  The  effect  would  be  the  same  in  altering  the 
apparent  direction  in  which  the  planet  would  be  seen  as 
if  he  were  looking  at  it  one  moment  from  Jamaica,  and 
then  were  suddenly  transported  to  see  it  from  Aden. 

The  difference  in  the  directions  in  which  the  planet  is 
seen  from  two  such  standpoints,  as  compared  with  the  posi- 
tions of  the  stars  around  it,  which  are  so  distant  that  no 
change  is  produced  in  their  apparent  places,  is  in  such  cases 
large  enough  to  be  capable  of  very  accurate  measurement, 
and  will  be  so  much  the  larger  and  more  easily  measurable 
the  nearer  any  planet  employed  is  to  the  earth. 

It  was  a  few  years  ago  supposed  that  the  diurnal  method 
would  prove  to  be  the  most  satisfactory  possible,  because  in 
it  the  same  observer  and  the  same  instrument  can  be  em- 
ployed for  all  the  observations.  We  are  inclined  to  think 
that  this  may  ultimately  prove  to  be  the  case  if  an  observa- 
tory suitably  equipped,  and  situated  near  to  the  equator, 
can  be  employed.  Dr.  Gill  has,  however,  introduced  such 
improvements  into  the  other  method  that  it  has  been 
chiefly  used  under  his  superintendence  for  the  last  pub- 
lished 2  and  most  accurate  result  that  has  yet  been  ob- 
tained— viz.,  from  observations  of  the  minor  planets  Vic- 
toria, Sappho,  and  Iris,  made  in  1888  and  1889  at  the  Cape 
of  Good  Hope,  and  at  Yale,  Leipsic,  Gottingen,  Bamberg, 
and  Oxford  (Radcliffe)  Observatories.  But  it  is  very  in- 
teresting that  it  is  also  found  that  observations  (nearly  3,500 
in  number)  made  at  the  same  time  by  the  diurnal  method 
upon  the  planet  Victoria  at  the  Cape,  although  that  ob- 
servatory is  unfavourably  situated  for  the  use  of  this 
method,  in  a  latitude  thirty-four  degrees  south  of  the 
equator,  gave  almost  precisely  the  same  result — a  value  of 
very  nearly  92,875,000  miles  as  the  distance  of  the  sun  from 
the  earth. 

This  is  a  very  trustworthy  value,  but  it  nevertheless  in- 
volves an  uncertainty  of  about  50,000  miles,  or  possibly 
2 "  Annals  of  the  Cape  Observatory,"  vol.  vi,  published  in  1897. 


THE   NEW  PLANET,   EROS  247 

somewhat  more.  It  was  obtained  by  the  observation  of 
planets  selected  for  their  suitability  of  position  and  because 
their  orbits  were  very  accurately  known,  which  did  not, 
however,  come  within  a  distance  equal  to  six  times  that  of 
the  nearest  approach  which  Eros  may  make  to  the  earth. 
There  is  every  reason,  therefore,  to  hope  that  our  future 
observations  of  Eros  may  give  us  this  all-important  unit 
for  all  our  celestial  measurings — the  distance  of  the  earth 
from  the  sun — with  an  accuracy  six  times  greater  than  any 
which  has  hitherto  been  secured. 

It  is  very  fortunate  that  Eros,  when  at  its  nearest  ap- 
proach to  the  sun,  is  also  almost  in  the  plane  of  the  earth's 
orbit.  If  the  earth  is  at  the  same  time  in  the  corresponding 
part  of  its  own  orbit,  the  planet's  approximation  to  the 
earth  is  consequently  in  nowise  hindered  by  its  being  ele- 
vated above,  or  depressed  below,  the  earth's  orbit.  But 
the  two  are  only  simultaneously  in  these  positions  once 
in  about  every  thirty  years,  and  it  is  very  unfortunate  that 
an  exceedingly  favourable  concurrence  of  such  positions 
occurred  in  January,  1894,  so  that  the  next  occasion  will 
not  be  until  January,  1924.  In  the  latter  part,  however, 
of  1900,  and  in  the  beginning  of  1901,  the  earth  and  Eros 
will  come  within  about  31,000,00x3  miles,  and  this,  their 
nearest  approach  to  one  another  before  the  year  1924, 
will  enable  observations  of  much  importance  to  be  made, 
which  ought  to  suffice  for  a  decided  improvement  in 
the  accuracy  of  our  present  estimate  of  the  distance  of 
the  sun. 

A  few  further  statements  with  reference  to  this  very  re- 
markable planet  may  be  of  interest.  Its  mean  distance  from 
the  sun  is  5,500,000  miles  less  than  that  of  Mars.  That 
distance  ranks,  therefore,  not  with  those  of  all  the  other 
little  planets  as  between  that  of  Mars  and  that  of  Jupiter, 
but  as  between  the  earth's  and  that  of  Mars.  Owing,  how- 
ever, to  the  ovalness  of  its  orbit,  it  passes  in  one  part  of 
its  circuit  about  11,000,000  miles  beyond  the  maximum  dis- 
tance of  Mars,  which  is,  nevertheless,  a  comparatively  slight 
excess.  Its  period  of  revolution  round  the  sun  is  643  days, 


248  LEDGER 

that  of  Mars  being  687.  There  is  no  fear  of  its  ever  col- 
liding with  Mars,  because  the  two  orbits,  where  they  would 
otherwise  intersect,  are  separated  by  an  interval  of  about 
21,000,000  miles,  owing  to  the  difference  of  their  tilts,  or 
inclinations  to  the  ecliptic. 

Besides  its  great  usefulness  for  the  purpose  already  ex- 
plained, the  perturbations  of  its  motion  by  the  earth's  at- 
traction will  afford  another  indirect  and  theoretically  very 
interesting  method  of  determining  the  distance  of  the  sun, 
by  its  enabling  a  comparison  to  be  made  between  the  masses 
of  the  earth  and  the  sun.  The  perturbations  of  the  motion 
of  Eros  by  Mars  will,  in  addition,  be  very  valuable  to 
astronomers.  Certain  recondite  effects  of  its  proximity 
in  reference  to  the  moon  may  also  prove  to  be  important. 
The  great  alterations  in  its  distance  from  the  earth  at  dif- 
ferent times  will  afford  an  excellent  test  as  to  whether  the 
light  received  from  it  varies  exactly  as  the  inverse  square 
of  its  distance  from  us,  or  meets  with  any  hindrance,  or 
absorbing  medium,  in  its  passage.  The  comparison  of  its 
light  with  the  phases  of  its  little  disk  corresponding  to  its 
positions  relatively  to  the  earth  and  the  sun  will  also  be 
instructive. 

The  discovery  of  Eros  has  afforded  a  most  important 
proof  of  the  value  of  stellar  photographs  carefully  kept  and 
preserved.  For  the  more  accurate  determination  of  the 
elements  of  its  orbit  it  was  very  desirable  to  obtain,  if  pos- 
sible, records  of  its  exact  position  in  previous  years.  Very 
careful  search  was  therefore  made  among  the  many  plates 
preserved  at  the  Harvard  College  Observatory,  in  America, 
in  order  to  see  if  its  faint  trace  could  be  found  upon  some 
of  those  which  had  been  exposed  in  1896.  It  seemed 
almost  impossible  to  detect  it.  But  at  last  success  rewarded 
a  search  which  proved  most  trying  to  the  eyes.  Mrs. 
Fleming,  well  known  for  her  splendid  work  in  connection 
with  stellar  spectra  in  the  Harvard  Observatory,  detected 
the  trail  upon  a  plate  dated  the  5th  of  June,  1896.  It  was 
soon  found  upon  other  plates  of  that  year,  as  its  probable 
position  could  not  be  more  precisely  calculated;  then  on 


THE   NEW   PLANET,   EROS  249 

others  of  1893  and  1894;  upon  thirteen  plates  in  all.  Its 
orbit  is  consequently  known  at  the  present  time  with  great 
accuracy. 

There  is  no  reason  to  suppose  that  Eros  is  a  body  re- 
cently drawn,  by  the  attraction  of  the  earth,  into  its  present 
orbit.  Its  near  approach  to  the  earth  is  by  no  means  near 
enough  for  such  an  event  as  that  to  have  occurred.  It  has 
doubtless  escaped  previous  observation  because  its  light 
has  only  exceeded  that  of  an  eighth-magnitude  star  (or,  for 
the  purpose  of  photography,  as  it  seems  to  be  wanting  in 
violet  rays,  that  of  a  ninth-magnitude  star)  for  about  two 
months  in  the  last  eleven  years.  On  the  comparatively  rare 
occasions  of  its  very  nearest  approach  it  will  be  barely 
within  the  range  of  visibility  to  the  naked  eye. 

In  conclusion,  it  may  be  noticed  that  the  proximity  of 
the  orbit  of  this  little  neighbour  to  that  of  the  earth  may 
afford  one  more  argument  against  the  hypothesis  put  for- 
ward by  Olbers  of  the  supposed  origin  of  the  minor  planets 
by  the  explosion  of  a  larger  planet,  a  hypothesis  which  for 
a  while  met  with  much  acceptance.  When  only  four  minor 
planets  had  been  discovered,  three  at  least  of  their  orbits 
tended  to  support  his  hypothesis.  It  has,  however,  since 
been  discarded  by  nearly  all  the  highest  authorities  in 
astronomy,  to  the  regret,  no  doubt,  of  those  whom  the  idea 
of  any  celestial  catastrophe  seems  to  fascinate,  whether  it  be 
the  possible  collision  of  two  suns,  the  destruction  of  the 
earth  by  a  comet,  or  the  blowing  of  a  planet  into  pieces 
by  its  own  inherent  forces.  Even  if  such  an  event  could 
have  occurred,  and  have  produced  the  minor  planets,  it 
must  have  been  at  an  exceedingly  remote  epoch;  other- 
wise, by  the  laws  of  mechanics  every  fragment  would  have 
continued,  once  in  each  of  its  subsequent  revolutions  round 
the  sun,  to  pass  again  and  again  through  the  point  of  ex- 
plosion. Millions  of  years  would  have  been  required  to 
enable  the  mutual  perturbing  attractions  of  the  fragments 
upon  each  other  so  to  change  their  orbits  as  to  have 
effaced  all  trace  of  the  point  where  the  catastrophe  took 
place.  Apart,  however,  from  this,  and  apart  from  the  fact 


250  LEDGER 

that  it  is  difficult  to  conceive  how  the  orbits  could  have 
been  spread  over  seven  eighths  of  the  vast  interval  which 
separates  those  of  Mars  and  Jupiter,  apart  also  from  the 
great  extension  of  the  region  in  which  they  move  which  is 
involved  in  the  newly  found  orbit  of  Eros,  another  argu- 
ment against  the  hypothesis  seems  to  be  conclusive. 

It  appears  impossible  to  conceive  of  such  an  amount  of 
explosive  energy  in  any  globe  as  should  not  only  hurl  away 
a  number  of  ejected  portions  in  the  directions  and  with  the 
velocities  which  would  produce  such  widely  differing  or- 
bits, but  such  as  should  break  up  and  suitably  project  the 
whole  mass  of  the  globe,  leaving  no  fragment  of  any  im- 
portance unprojected.  A  cannon-ball,  as  Proctor  has  re- 
marked in  his  "  Old  and  New  Astronomy,"  "  might  be 
driven  by  a  certain  charge  of  gunpowder  to  a  distance  of 
two  or  three  miles,  but  a  thousand  times  that  charge  would 
not  scatter  the  fragments  of  the  cannon  (if  the  ball  had  been 
tightly  driven  in)  over  a  similar  distance  all  round  the  place 
of  explosion.  Nothing  known  about  our  earth's  interior, 
nothing  which  we  can  infer  about  the  interior  of  any  other 
planet  formed  by  processes  such  as  we  recognise  in  the  de- 
velopment of  the  solar  system  as  at  present  understood, 
suggests  the  possibility  that  a  millionth  part  of  the  force 
necessary  to  shatter  a  planet,  as  Olbers's  theory  requires, 
can  ever  be  generated  or  accumulated  within  the  planet's 
interior."  3 

Rather  may  we  see  in  a  planet  such  as  Eros  a  portion  of 
the  primeval  solar  nebula  unused  in  the  formation  either  of 
Mars  or  of  the  earth.  The  minor  planets  are  probably  no 
fragments  of  a  larger  planet  previously  existing,  but  the 
fragments  that  might  have  helped  to  form  a  larger  planet 
had  it  not  been  for  the  influence  of  the  mighty  globe  of 
Jupiter.  We  may  see  in  them  one  more  instance  of  the 
effect  of  that  process  of  tidal  action  which  Professor  Darwin 
has  of  late  so  wonderfully  applied  to  show  how  the  matter 
of  the  moon  may,  in  bygone  time,  have  been  disrupted  from 
the  then  viscous  earth,  in  the  form  of  a  succession  of  lumps 
8 "  Old  and  New  Astronomy,"  p.  563. 


THE   NEW  PLANET,   EROS  251 

broken  off  by  centrifugal  effect  from  the  summits  of  great 
tidal  waves — a  hypothesis  which  is  found  to  be  of  ever- 
widening  application,  as,  for  instance,  to  the  genesis  of 
double  stars,  and  to  the  temporary  outburst  of  such  stars 
as  that  which  Kepler  saw  in  1572  in  Cassiopeia. 

The  attraction  of  the  globe  of  Jupiter,  as  the  solar 
nebula  contracted  within  his  orbit,  may  well  have  produced 
such  tides  in  its  mass  as,  in  place  of  allowing  a  greater 
quantity  of  matter  or  a  nebulous  ring  to  be  more  quietly 
detached  at  some  subsequent  epoch,  so  as  to  form  another 
large  globe,  may  have  caused  many  and  many  a  smaller 
portion  to  have  been  broken  off  and  left  behind.  These 
portions  we  may  now  see  in  the  hundreds  of  minor  planets 
which  have  so  far  been  discovered.  After  a  while,  it  may 
be  supppsed  that  the  influence  of  Jupiter  was  so  far  left 
behind  by  the  continued  contraction  of  the  solar  nebula  that 
the  formation  of  larger  globes,  such  as  those  of  Mars  and 
the  earth,  Venus  and  Mercury,  began  again. 

However  this  may  be,  let  us  hope  that  in  the  succession 
of  celestial  photographs  now  being  continuously  secured 
other  similar  fragments  may  ere  long  be  revealed  whose 
orbits  may  be  as  interesting  as  that  of  Eros,  whether  they 
may  revolve  within,  or,  like  it,  outside  of  the  orbit  of  the 
earth.  Let  us  hope  that  some  of  them  may  approach  the 
earth  even  more  closely  than  Eros.  If  so,  they  will  be  still 
more  useful  rewards  of  the  unwearied  industry  of  observers 
and  computers,  and  of  the  skill  displayed  in  astronomical 
photography. 


SIDEREAL  ASTRONOMY: 
OLD  AND  NEW 

PHOTOGRAPHY  THE  SERVANT 
OF  ASTRONOMY 

THE  BEGINNINGS  OF  AMERICAN 
ASTRONOMY 


BY 

EDWARD   SINGLETON   HOLDEN 


SIDEREAL  ASTRONOMY:  OLD  AND  NEW1 

I.    THE   DATA   IT  HAS  COLLECTED 

WHEN  did  astronomy  have  its  beginnings  on  the 
earth?    There  have  been  many  learned  attempts 
to  answer  this  question.    They  all  have  led  to  the 
conclusion  that  long  before  the  historic  period  there  was 
a  large  common  stock  of  knowledge;  so  large,  in  fact,  that 
one  distinguished  writer  finds  it  simplest  to  ascribe  the 
origin  of  astronomy  to  the  teaching  of  an  extinct  race:  "  Ce 
peuple  ancien  qui  nous  a  tout  appris — excepte  son  nom  et 
son  existence,"  his  commentator  adds. 

Astronomy  is  older  than  the  first  records  of  any  nation. 
In  order  that  the  records  might  exist,  it  was  first  necessary 
to  divide  the  years  and  times  by  astronomical  observations. 
On  the  other  hand,  I  believe  the  travellers  of  to-day  have 
found  no  tribe  so  degraded  as  to  be  without  some  knowl- 
edge of  the  sort. 

It  is  extremely  doubtful  if  animals  notice  special  celes- 
tial bodies.  Birds  seem  to  be  inspired  by  the  approach  of 
day  and  not  by  the  actual  presence  of  the  sun.  It  is  a  ques- 
tion whether  dogs  "  bay  the  moon  "  or  only  the  moon's 
light.  A  friend  maintains  that  her  King  Charles  spaniel 
watched  the  progress  of  an  occupation  of  Venus  by  the 
crescent  moon  with  the  most  vivid  interest.  This  is  the 
only  case  which  I  have  been  able  to  collect  in  which  the 
attention  of  animals  has  been  even  supposed  to  have  been 
held  by  a  celestial  phenomenon.  The  actions  of  the  most 
ignorant  savages  during  a  total  solar  eclipse,  compared  with 

1  From  the  "  Century  Magazine,"  August  and  September,  1888.    By 
special  permission  of  the  Century  Company. 

255 


256  HOLDEN 

those  of  animals,  throw  much  light  on  the  question  of 
whereabouts  in  the  scale  of  intelligence  the  attention  begins 
to  be  directed  to  extra-terrestrial  occurrences.  The  savages 
are  appalled  by  the  disappearance  of  the  sun  itself,  while 
animals  seem  to  be  concerned  with  the  advent  of  darkness 
simply. 

I  am  told  that  the  Eskimos  of  Smith's  Sound  have 
names  for  a  score  or  more  of  stars,  and  that  their  long 
sledge  journeys  are  safely  made  by  the  guidance  of  these 
stars  alone.  I  have  myself  seen  a  Polynesian  islander  em- 
bark in  a  canoe,  without  compass  or  chart,  bound  for  an 
island  three  days'  sail  distant.  His  course  would  need  to 
be  so  accurately  laid  that  at  the  end  of  his  three  days  he 
should  find  himself  within  four  or  five  miles  of  his  haven; 
if  he  passed  the  low  coral  island  at  a  greater  distance  it 
could  not  be  seen  from  his  frail  craft.  There  can  be  little 
doubt  but  that  he  used  the  sun  by  day  and  the  stars  by 
night  to  hold  his  course  direct. 

There  must  have  been  centuries  during  which  such 
knowledge  was  passed  from  man  to  man  by  word  of  mouth, 
woven  into  tales  and  learned  as  a  part  of  the  lore  of  the 
sailor,  the  hunter,  or  the  tiller  of  the  soil.  No  one  can  say 
how  early  this  knowledge  of  the  sky  was  put  into  the  for- 
mal shape  of  maps,  globes,  or  catalogues.  Eudoxus  is  said 
to  have  constructed  a  celestial  globe  B.  c.  366.  Globes  would 
naturally  precede  maps,  and  maps  mere  lists  or  catalogues. 

The  prototype  of  all  sidereal  catalogues  is  the  "  Alma- 
gest "  of  Ptolemy  (A.  D.  150),  which  includes  not  only  the 
observations  of  Ptolemy,  but  those  of  the  great  Hippar- 
chus  (B.  c.  127).  It  contains  the  description  of  1,022  stars, 
their  positions,  and  their  brightness.  Here  we  meet  for 
the  first  time  the  name  magnitude  of  a  star.  Ptolemy 
divides  all  the  stars  into  magnitudes — degrees  of  bright- 
ness. Sirius,  Capella,  are  of  the  first  magnitude;  the  faint- 
est stars  visible  to  the  eyes  are  of  the  sixth.  But  Ptolemy 
has  gone  further,  and  divides  each  magnitude  into  three 
parts.  The  moderns  divide  each  class  into  ten  parts — that 
is,  decimally. 


SIDEREAL  ASTRONOMY:  OLD   AND   NEW          257 


SCALE   OF   MAGNITUDES 

In  assigning  magnitudes  in  this  way,  we  have  uncon- 
sciously adopted  a  scale.  A  star  of  the  third  magnitude  is 
brighter  than  one  of  the  fourth.  How  much  brighter? 
Sirius  and  the  brightest  stars  are  about  one  hundred  times 
more  brilliant  than  the  very  faintest  stars  which  can  be  seen 
with  the  naked  eye.  In  general  a  star  of  any  magnitude, 
as  fifth,  is  four  tenths  as  bright  as  the  star  of  the  next 
brighter  magnitude,  as  fourth.  Ten  fifth-magnitude  stars 
taken  together  are  as  bright  as  four  fourth-magnitude  stars, 
and  so  on.  This  relation  between  the  brightness  of  stars 
of  consecutive  magnitudes  gives  us  a  means  of  computing 
the  total  amount  of  light  received  from  stars.  For  example, 
there  are  ten  stars  in  our  sky  as  bright  as  the  brilliant  star 
Vega,  or  Alpha  Lyrse,  which  we  see  in  our  zenith  during 
the  summer  months.  The  collective  light  of  these  ten  first- 
magnitude  stars  is  10  times  that  of  Vega.  The  37  second- 
magnitude  stars  are  together  7.4  times  as  bright  as  Vega; 
the  128  third-magnitude  stars  are  10.2  times  as  bright;  and 
so  on  down  to  the  4,328  sixth-magnitude  stars,  which, 
taken  together,  are  22.1  times  as  bright.  Taking  all  the 
stars  visible  to  us  without  a  telescope  and  adding  their 
brilliancy,  we  find  that  all  the  naked-eye  stars  give  us  a 
light  67.6  times  as  bright  as  that  from  Vega.  Now,  the 
stars  of  the  seventh  and  eighth  magnitudes  have  been 
counted;  there  are  13,593  of  the  seventh,  57,960  of  the 
eighth,  and  they  too  send  light  to  us,  although  they  are 
individually  invisible.  All  the  seventh-magnitude  stars 
taken  together  give  us  27.8  times  as  much  light  as  Vega, 
and  the  eighth  give  us  47.4  as  much;  so  that  we  have  from 
both  of  these  classes  75.2  times  the  light  of  Vega — that  is, 
more  light  comes  to  us  from  stars  so  faint  as  to  be  indi- 
vidually invisible  than  from  the  less  numerous  and  brighter 
stars  that  we  see  with  the  naked  eye.  We  may  recollect  that 
more  than  half  of  the  light  of  a  starlit  night  comes  from  the 
collective  lustre  of  stars,  each  of  which  is  totally  invisible 
except  in  the  telescope. 
17 


258  HOLDEN 

METHODS    OF   NAMING   THE    STARS 

In  Ptolemy's  "  Almagest,"  and  for  fifteen  centuries 
later,  there  were  two,  and  but  two,  ways  of  designating  a 
particular  star.  A  few  of  the  brighter  stars  had  special 
names. 

By  far  the  greater  number  were  described  by  their  situa- 
tion in  their  constellation.  The  brightest  star  in  Taurus  was 
the  eye  of  the  Bull,  and  so  for  others,  as  the  belt  and  sword 
of  Orion.  This  was  all  very  well  for  the  brighter  stars,  and 
it  did  not  require  that  the  boundaries  of  the  constellations 
should  be  very  accurately  fixed.  There  was  no  mistaking 
Regulus,  Cor  Leonis — the  heart  of  the  lion.  But  when  we 
come  to  the  small  pairs  of  stars  which  make  the  paws  of  the 
Great  Bear,  or  to  some  of  the  stars  in  the  windings  of  Ser- 
pens,  then  it  is  evident  that  Ptolemy  must  have  had  ac- 
curately bounded  constellations  laid  down  on  charts  or 
globes.  Not  a  single  ancient  globe  or  chart  has  come 
down  to  us.  The  oldest  extant  are  but  Arabian  copies  of 
the  tenth  century. 

Where,  then,  do  we  derive  our  figures  of  the  constella- 
tions? If  any  one  of  my  readers  will  ask  some  astronomical 
friend  to  show  him  a  copy  of  Flamsteed's  "Atlas  Ccelestis  " 
he  will  see  the  beautiful  and  spirited  drawings  of  the  con- 
stellation figures,  and  be  charmed  and  delighted  with  their 
vigour  and  character.  Who  could  have  drawn  these  out- 
lines, instinct  with  life?  Who  of  the  ancients  knew  the 
whole  character  of  the  timid  hare,  or  who  could  draw 
Andromeda,  and  put  a  modern  resignation  in  her  chained 
despair?  These  figures  were  drawn  by  a  master  indeed,  for 
they  are  from  the  hand  of  Albert  Diirer  himself.  If  we  fol- 
low the  history  of  how  he  came  to  make  them  for  an  edi- 
tion of  Ptolemy,  and  think  of  him  patiently  fitting  his  mar- 
vellously free  outlines  to  match  the  stars  in  the  sky  and 
the  crabbed  descriptions  in  Ptolemy's  book,  the  pleasure 
does  not  dimmish.  About  1603  Bayer  introduced  the  prac- 
tice of  designating  the  brighter  stars  of  each  constellation 
by  the  letters  of  the  Greek  alphabet,  so  that  Cor  Leonis  or 


SIDEREAL  ASTRONOMY:  OLD  AND  NEW          259 

Regulus  became  a  Leonis;  Aldebaran  became  a  Tauri,  and 
so  on.  As  the  number  of  the  well-determined  stars  has 
vastly  increased,  the  practice  of  referring  to  them  by  their 
numbers  in  some  well-known  catalogue  has  come  into 
vogue;  so  that  a  Leonis,  for  example,  might  be  known  as 
Bradley,  1406,  from  its  number  in  Bradley 's  catalogue; 
or  as  Lalande,  19,755,  an<^  so  on-  ^  *s  not  to  be  denied 
that  astronomical  nomenclature  in  this  direction  could  be 
greatly  improved. 

URANOMETRIES 

The  word  uranometry  has  received  a  limited  technical 
meaning  in  astronomy.  It  is  used  to  denote  a  description 
of  the  fixed  stars  which  are  visible  to  the  naked  eye  only. 
The  description  of  each  star  places  it  in  its  proper  con- 
stellation, assigns  its  latitude  and  longitude,  and  gives  its 
brightness  or  magnitude.  Variable  stars,  which  change 
their  brightness  periodically — and  there  are  many  such — 
are  treated  separately. 

Ptolemy's  "  Almagest  "  (1,022  stars)  was  an  incomplete 
uranometry,  since  there  were  more  than  3,000  stars  visible 
to  him.  Al-Sufi's  revision  of  it,  in  the  tenth  century,  added 
no  stars,  but  simply  revised  the  magnitudes  given  by  Ptol- 
emy. Bayer  (1603)  gave  1,200  stars.  None  of  the  very 
important  works  of  Flamsteed  (1753),  Harris  (1725),  Wol- 
laston  (1811),  Harding  (1822),  were  complete — that  is,  no 
one  gave  every  star  down  to  a  certain  brightness.  It  was 
reserved  for  Argelander  (1843)  to  give  in  the  "  Uranometria 
Nova"  the  position  of  brightness  of  every  star  visible  to  the 
naked  eye  at  Bonn.  This  was  a  picture  of  the  sky;  changes 
could  no  longer  occur  without  detection.  This  work  gave 
the  places  of  3,256  stars,  from  first  to  sixth  magnitudes,  and 
very  careful  eye-estimates  of  their  magnitudes.  Argelan- 
der's  work  has  been  repeated  by  Heis  (1872).  The  southern 
sky  has  been  treated  in  the  same  way  by  Dr.  Gould,  in  the 
"  Uranometria  Argentina"  (1879),  containing  6,694  south- 
ern and  991  northern  stars,  of  magnitudes  between  the  first 
and  seventh.  Houzeau,  during  a  residence  in  Jamaica, 


260  HOLDEN 

made  a  uranometry  which  embraces  every  star  in  both 
hemispheres,  and  which  has  a  special  value  owing  to  the 
fact  that  the  estimates  of  magnitude  were  all  made  by  a 
single  person,  and  are  therefore  consistent. 

We  have,  then,  a  complete  picture  of  our  sky,  as  seen 
with  the  naked  eye,  based  on  eye-estimates  of  the  bright- 
ness of  the  stars.  It  should  be  said  that  the  magnitudes  so 
determined  are  extremely  accurate,  approaching  closely  to 
the  exactness  which  can  be  reached  with  the  best  pho- 
tometers, or  instruments  for  measuring  the  relative  bright- 
ness of  stars. 


Up  to  1877,  when  Professor  Pickering  became  director 
of  the  Harvard  University  Observatory,  there  was  no  single 
observatory  devoted  to  photometry  as  a  chief  end.  The 
important  works  of  this  nature  had  been  done  as  a  part 
of  other  duties.  Professor  Pickering  turned  the  whole 
strength  of  the  observatory  in  this  direction,  and  by  means 
of  new  methods  and  new  instruments  he  and  his  assistants 
have  just  completed  a  work  of  the  first  importance — the 
"  Harvard  Photometry."  It  contains  the  positions  and 
the  measured  brightness  of  4,260  stars  visible  at  Cam- 
bridge, together  with  a  comparison  with  the  magnitudes 
of  all  other  observers.  The  actual  number  of  single  ob- 
servations is  95,000.  Each  one  of  these  consists  in  a  direct 
photometric  comparison  of  the  relative  brightness  of  a  star 
with  one  of  the  polar  stars.  The  polar  stars  are  always 
visible;  the  stars  to  be  measured  were  taken  as  they  crossed 
the  meridian;  and  these  direct  measures,  suitably  com- 
bined, give  the  relative  brightness  of  each  of  the  stars  of  the 
list.  We  have  now  a  sure  basis  for  all  future  work,  and  a 
perfect  picture  of  the  sky  at  this  time. 

THE  NUMBER  OF  THE  STARS 

The  total  number  of  stars  one  can  see  will  depend  very 
largely  upon  the  clearness  of  the  atmosphere  and  the  keen- 
ness of  the  eye.  There  are  in  the  whole  celestial  sphere 


SIDEREAL  ASTRONOMY:    OLD   AND  NEW          261 

about  6,000  stars  visible  to  an  ordinarily  good  eye.  Of 
these,  however,  we  can  never  see  more  than  a  fraction  at 
any  one  time,  because  a  half  of  the  sphere  is  always  below 
the  horizon.  If  we  could  see  a  star  in  the  horizon  as  easily 
as  in  the  zenith,  a  half  of  the  whole  number,  or  3,000,  would 
be  visible  on  any  clear  night.  But  stars  near  the  horizon 
are  seen  through  so  great  a  thickness  of  atmosphere  as 
greatly  to  obscure  their  light,  and  only  the  brightest  ones 
can  there  be  seen.  As  a  result  of  this  obscuration,  it  is  not 
likely  that  more  than  2,000  stars  can  ever  be  taken  in  at 
a  single  view  by  any  ordinary  eye.  About  2,000  other  stars 
are  so  near  the  south  pole  that  they  never  rise  in  our  lati- 
tudes. Hence,  out  of  6,000  supposed  to  be  visible,  only 
4,000  ever  come  within  the  range  of  our  vision,  unless  we 
make  a  journey  toward  the  equator. 

As  telescopic  power  is  increased,  we  still  find  stars  of 
fainter  and  fainter  light.  But  the  number  can  not  go  on 
increasing  forever  in  the  same  ratio  as  with  the  brighter 
magnitudes,  because,  if  it  did,  the  whole  sky  would  be  a 
blaze  of  starlight.  If  telescopes  with  powers  far  exceeding 
our  present  ones  were  made,  they  would  no  doubt  show 
new  stars  of  the  twentieth  and  twenty-first,  etc.,  magni- 
tudes. But  it  is  highly  probable  that  the  number  of  such 
successive  orders  of  stars  would  not  increase  in  the  same 
ratio  as  is  observed  in  the  eighth,  ninth,  and  tenth  magni- 
tudes, for  example.  The  enormous  labour  of  estimating 
the  number  of  stars  of  such  classes  will  long  prevent  the 
accumulation  of  statistics  on  this  question;  but  this  much 
is  certain,  that  in  special  regions  of  the  sky,  which  have 
been  searchingly  examined  by  various  telescopes  of  suc- 
cessively increasing  apertures,  the  number  of  new  stars 
found  is  by  no  means  in  proportion  to  the  increased  instru- 
mental power.  If  this  is  found  to  be  true  elsewhere,  the 
conclusion  may  be  that,  after  all,  the  stellar  system  can  be 
experimentally  shown  to  be  of  finite  extent  and  to  contain 
only  a  finite  number  of  stars.  In  the  whole  sky  an  eye  of 
average  power  will  see  about  6,000  stars,  as  I  have  just 
said.  With  a  telescope  this  number  is  greatly  increased, 


262 


HOLDEN 


and  the  most  powerful  telescopes  of  modern  times  will  show 
more  than  60,000,000  stars.  Of  this  number  not  one  out 
of  one  hundred  has  ever  been  catalogued  at  all. 

In  Argelander's  "  Durchmusterung  "  of  the  stars  of  the 
northern  heavens,  there  are  recorded  as  belonging  to  the 
northern  hemisphere: 


io  stars 

37 

128 

310 

1,016 

4,328 

13,593 

237,544 


between  the  i.o  magnitude  and 
2.0 
3.0 
4.0 
5-0 
6.0 
7.0 
8.0 
9.0 


the  1.9  magnitude. 
2.9 

3-9 
4.9 

5-9 
6.9 

7-9 
8.9 

9-5 


In  all,  314,926  stars,  from  the  first  to  the  9j  magnitudes, 
are  contained  in  the  northern  sky,  or  about  600,000  in  both 
hemispheres.  All  of  these  can  be  seen  with  a  three-inch 
object-glass. 

THE   CHARTS   OF  THE   BERLIN   ACADEMY 

In  1824  Bessel  wrote  to  the  Academy  of  Berlin  some- 
what as  follows: 

"  It  is  of  the  highest  astronomical  interest  that  every 
fixed  star  in  the  sky  should  be  known,  and  its  position 
fixed.  Completeness  in  this  task  is  unattainable;  but  when 
we  once  have  maps  of  all  the  stars  down  to  a  certain  mag- 
nitude, then  the  object  will  be  attained.  The  limit  I  set  is 
at  those  stars  which  can  just  be  plainly  seen  in  one  of 
Fraunhofer's  excellent  comet-seekers  2 — that  is,  at  about 
the  ninth  or  tenth  magnitude." 

Bessel  then  gives  briefly  the  reasons  why  such  a  com- 
plete list  would  be  valuable,  in  addition  to  its  importance 
as  a  finished  picture  of  the  sky  so  far  as  it  went;  and  con- 
tinues: 

"  For  all  these  reasons  I  have  often  expressed  my  hope 
that  we  might  have  such  a  complete  list,  if  even  over  only 
a  portion  of  the  sky;  and  I  think  the  time  of  an  astronomer, 
*  A  telescope  with  about  three  inches  aperture,  magnifying  ten  times. 


SIDEREAL   ASTRONOMY:    OLD  AND   NEW          263 

and  of  an  observatory,  could  not  be  better  spent  than  in 
aiding  a  systematic  attempt  to  carry  out  this  plan.  I  my- 
self designed  the  instruments  of  the  Konigsberg  Observa- 
tory for  such  a  purpose,  and  since  1821  I  have  observed 
as  many  as  possible  of  the  stars  from  15°  north  to  15°  south 
of  the  equator.  In  all  there  are  36,000  observations  of 
32,000  stars.  If  the  stars  are  equally  numerous  over  the 
whole  sky,  there  are  125,000  such.  I  am  about  to  carry  on 
these  zones  up  to  45°  from  the  equator." 

With  this  introduction  Bessel  unfolds  his  plan,  which 
was  to  have  24  astronomers  join  in  an  undertaking  to 
make  the  24  separate  charts  required  to  extend  round  the 
whole  24  hours,  and  in  width  over  the  30°  from  15°  north 
to  15°  south  of  the  equator.  He  himself  made  a  small  chart 
as  a  beginning,  "  to  break  the  path,"  and  as  a  model.  The 
Academy  welcomed  Bessel's  plan,  and  the  work  began  in 
1825. 

The  first  two  charts  were  received  in  1828,  and  the 
work  on  the  others  continued  slowly.  One  of  these  charts 
has  a  great  history.  It  had  been  engraved  but  not  yet  dis- 
tributed, and  was  lying  in  the  Berlin  Observatory  for  ex- 
amination. On  the  evening  of  September  23,  1846,  Le 
Verrier's  letter,  giving  the  place  of  a  new  planet,  Neptune, 
was  received  in  Berlin.  The  planet  had  never  been  seen, 
but  its  existence  had  been  predicted  from  the  otherwise  in- 
explicable motions  of  Uranus.  The  predicted  place  of  the 
planet  fell  within  the  limit  of  the  lately  finished  chart,  which 
was  taken  to  the  telescope.  In  very  truth  there  was  an 
eighth-magnitude  star  in  the  sky  which  was  not  on  the 
chart.  This  star  was  in  motion;  it  had  the  planetary  light 
and  disk;  it  was,  in  fact,  Neptune.  The  proposal  of  Bessel 
had  borne  splendid  fruit.  Besides  this  major  planet,  many 
of  the  minor  planets  (asteroids)  were  discovered  by  these 
maps.  Finally,  in  1859,  thirty-five  years  after  Bessel's  let- 
ter, this  series  was  finished.  But  before  it  was  finished  a 
greater  undertaking  was  begun,  of  which  we  must  give  a 
short  account.  One  thing  must  be  continually  kept  in 
sight.  Every  one  of  the  systematic  "  Durchmusterungen," 


264  HOLDEN 

as  the  Germans  say — we  have  no  word  for  them — is  the 
direct  outcome  of  Bessel's  original  proposition. 

ARGELANDER'S  "  DURCHMUSTERUNG  " 

Argelander  was  Bessel's  pupil.  In  the  great  zones  of 
Konigsberg,  Bessel  had  pointed  the  telescope  on  the  stars 
as  they,  passed,  and  Argelander  read  the  verniers  which 
showed  their  position.  Finally,  Argelander  had  an  observa- 
tory of  his  own  at  Bonn,  and  his  two  young  assistants, 
Drs.  Krueger  and  Schoenfeld,  were  all  to  him  that  he  had 
been  to  Bessel.  The  years  1852  to  1862  were  spent  in  the 
tremendous  task  of  observing  every  star  plainly  visible  in 
such  a  comet-seeker  as  we  have  described,  over  more  than 
half  of  the  whole  heavens.  The  telescope  was  pointed  and 
fixed  in  position.  The  time  of  the  passage  of  every  star 
over  a  wire  in  the  field  of  view  was  noted;  the  part  of  the 
wire  crossed  by  the  star  was  also  noted,  and  finally  the 
brightness  of  the  star. 

Not  counting  the  time  for  the  computations,  the  ob- 
servations alone  lasted  seven  years  and  one  month;  1,797 
hours  were  spent  in  observing  the  comet-seeker  zones  on 
625  nights;  and  227  other  nights  were  used  in  part  or 
wholly  in  revision  zones  to  correct  errors  of  one  nature  or 
another,  or  to  solve  doubts. 

In  the  comet-seeker  zones  850,000  single  observations 
were  made,  or  on  the  average  473  stars  per  hour,  or  8  per 
minute.  In  specially  rich  parts  of  the  Milky  Way  more 
than  1 6  stars  per  minute  were  often  observed,  and  the  rich- 
est zone  had  1,226  stars  in  the  hour,  or  20^  per  minute — 
one  every  3  seconds.  Counting  all  the  observations  to- 
gether, there  were  no  less  than  1,065,000,  and  this  million 
of  observations  gave  the  positions  and  the  brightness  of 
324,198  stars — that  is,  the  position  and  brightness  of  every 
star  plainly  visible  in  the  telescope  used,  from  the  north 
pole  down  to  2°  south  of  the  equator. 

The  very  enumeration  of  the  observations  makes  one 
fatigued.  Only  the  astronomer  can  know  the  multifarious 
nature  of  the  calculations  connected  with  the  observations 


SIDEREAL  ASTRONOMY:    OLD  AND  NEW          265 

themselves.  Millions  on  millions  of  figures  had  to  be  made, 
and  made  correctly;  and,  finally,  every  star  had  to  be  en- 
graved on  charts,  and  engraved  correctly  both  as  to  posi- 
tion and  magnitude. 

How  this  work  could  have  been  finished  in  ten  years 
one  does  not  see.  That  Argelander  and  his  two  assistants 
had  the  courage  to  persevere  in  this  tremendous  task  is 
itself  a  marvel.  But  the  work  is  done,  is  printed,  and  is  in 
daily  use  by  scores  of  astronomers.  Its  value  will  never  be 
less.  It  will  remain  forever  as  a  picture  of  the  sky,  avail- 
able for  every  purpose. 

Mr.  Proctor  has  done  a  very  useful  wrork  in  represent- 
ing the  results  of  Argelander's  "  Durchmusterung  "  in  a 
single  chart.  For  every  star  in  Argelander's  catalogue 
Mr.  Proctor  has  laid  down  a  dot,  correct  as  to  position  and 
magnitude — 324,198  dots  in  all.  The  resulting  map  is 
photographed  down  so  that  the  individual  dots  are,  in 
general,  hard  to  distinguish,  but  the  law  of  aggregation  of 
the  stars  is  all  the  better  brought  out.  The  map  is  most 
interesting,  not  only  in  relation  to  the  mere  positions  and 
brilliancy  of  the  stars,  but  as  showing,  better  than  any 
other  means  can,  the  apparently  capricious  manner  in 
which  the  stars  are  spread  over  the  surface  of  the  sky. 
Some  evidences  of  law  can  be  made  out,  and,  in  the  origi- 
nal, the  great  features  of  the  Milky  Way  come  forth  in 
a  most  striking  manner.  It  must  be  remembered  that  this 
map  contains,  besides  the  stars  visible  to  the  naked  eye,  all 
those  visible  in  an  ordinary  three-inch  telescope. 

SCHOENFELD'S  "  DURCHMUSTERUNG  " 
Argelander's  original  plan  was  to  extend  his  observa- 
tions to  23°  south  of  the  equator.  Professor  Schoenfeld, 
his  successor  at  Bonn,  and  his  aid  in  the  original  under- 
taking, in  1885  completed  the  plan  projected  by  Bessel  in 
1824,  and  so  nobly  followed  at  Bonn  from  1852  to  1860. 
From  1876  to  1884  he  has  catalogued  the  stars  from  2° 
to  23°  south  of  the  equator,  and  the  work  is  just  finished. 
Soon  we  shall  have  this  new  "  Durchmusterung,"  with  its 


266  HOLDEN 

charts,  showing  the  position  and  brightness  of  133,658 
southern  stars. 

It  is  most  desirable  that  this  enumeration  should  be 
extended  over  the  whole  southern  sky.  So  long  ago  as 
1866  the  work  was  begun  in  the  southern  hemisphere,  but 
apparently  it  was  abandoned,  though  there  is  reason  to 
believe  that  the  observatory  of  the  Argentine  Republic  at 
Cordoba  may  begin  anew.  Professor  Stone,  at  Cincinnati, 
has  partly  completed  the  zone  between  23°  and  31°  (south). 

A  recognition  of  the  enormous  advantages  which  pho- 
tography would  have  over  ordinary  visual  methods  of  chart- 
ing is  now  leading  several  observatories  to  attempt  the 
cataloguing  of  stars  from  photographic  negatives. 

The  difficulties  are  many,  but  success  seems  to  be  tol- 
erably certain,  and  the  observatories  of  Harvard  University 
and  of  Paris  have  already  produced  wonderful  results  in 
this  direction.  The  observatory  of  the  Cape  of  Good  Hope, 
also,  has  seriously  begun  a  southern  "  Durchmusterung  " 
by  photographic  methods. 

SYSTEMATIC  OBSERVATORIES  OF  THE  STARS  IN  ZONES 

These  "  Durchmusterungen "  are  most  important. 
They  give  us  an  index  to  the  stars  of  the  whole  sky.  But 
it  is  clear  that  the  positions  of  the  separate  stars  can  not 
be  accurate  when  so  many  as  eight  or  ten  per  minute  are 
observed.  What  the  astronomer  wants  is  the  accurate  posi- 
tion of  a  star — its  latitude  and  longitude,  as  it  were.  We 
shall  see  how  much  pains  is  necessary  to  fix  the  position  of 
a  single  star  with  real  precision.  Scores  of  observations  are 
needed,  and  each  observation  requires  at  least  five  minutes 
to  make  and  an  hour  to  calculate.  When  we  say  that  many 
thousand  stars  have  their  positions  known  with  this  high 
precision,  we  shall  be  giving  a  feeble  idea  of  the  amount 
of  labour  devoted  to  this  question. 

But  it  is  impossible  to  fix  the  position  of  every  one  of 
the  600,000  stars  of  the  "  Durchmusterungen  "  with  this 
last  degree  of  precision,  and  yet  it  is  important  to  know 
very  closely  the  place  of  each  star.  The  positions  of  all  faint 


SIDEREAL  ASTRONOMY:    OLD   AND  NEW          267 

comets,  of  asteroids,  etc.,  are  known  by  referring  them  to 
neighbouring  stars.  We  must  know  the  positions  of  these 
stars.  These  positions  are  determined  by  a  special  kind 
of  observations — zone  observations,  so  called.  A  telescope 
is  fixed  in  the  meridian  so  that  it  can  only  move  north 
and  south.  A  divided  circle  is  attached  to  this,  the  indica- 
tions of  which  give  the  altitude  of  the  stars  seen  in  the  field. 
One  observer  at  the  telescope  moves  it  slowly  up  and  down 
until  some  star  enters  the  field.  The  motion  is  stopped. 
The  transit  of  the  star  is  observed  over  spider  lines  stretched 
in  the  field,  while  a  second  observer  reads  the  altitude  of 
this  star  from  the  divided  circle.  In  this  way  it  is  possible 
to  obtain  very  accurate  positions,  and  by  confining  the 
work  to  a  narrow  zone  the  observations  are  increased  as 
to  number,  and  the  subsequent  computations  are  much 
simplified. 

Before  the  days  of  the  Berlin  charts,  or  of  the  "  Durch- 
musterungen,"  Lalande,  in  Paris  (1870),  had  fixed  the 
places  of  more  than  50,000  stars  in  this  way,  and  the  Abbe 
Lacaille(i75i)had  made  a  special  expedition  to  the  Cape  of 
Good  Hope  to  determine  the  places  of  9,766  southern  stars. 
Bessel  took  up  the  same  research  in  the  years  1821-^33,  and 
his  results  are  given  in  two  magnificent  catalogues,  which 
include  62,000  of  the  most  important  stars  from  15°  south 
to  45°  north  of  the  equator.  He  made  75,011  single  ob- 
servations, employing  868  hours  in  observing  alone — that 
is,  about  84  stars  per  hour  were  observed.  Argelander 
read  the  altitudes  of  the  stars  from  the  circle  while  Bessel 
observed  their  transits.  One  of  Argelander's  first  works, 
when  he  took  charge  of  the  observatory  at  Bonn,  was  to 
continue  this  series  of  zones  from  45°  up  to  80°  north  of 
the  equator — that  is,  to  within  10°  of  the  pole.  In  this 
region  he  made  26,424  observations  of  22,000  stars,  or  83 
stars  per  hour.  Not  content  with  this  extension  of  Bessel's 
zones  to  the  north,  Argelander  next  began  a  series  of 
southern  zones  from  15°  to  31°  south  of  the  equator.  This 
task  he  also  completed,  with  23,250  observations  of  17,600 
stars,  or  83  stars  per  hour. 


268  HOLDEN 

Bessel  and  Argelander  alone  had  pushed  their  zones 
from  31°  south  to  80°  north  of  the  equator,  making  nearly 
125,000  separate  observations  and  fixing  the  positions  of 
101,600  stars.  We  have  no  space  to  speak  of  the  38,000 
observations  made  at  the  Naval  Observatory  in  Washing- 
ton in  the  years  1846-^49,  or  of  the  zones  observed  by 
Lieutenant  Gilliss,  of  our  navy,  in  Chile  (1850),  which  cov- 
ered the  region  for  25°  round  the  south  pole  (27,000  stars). 
It  is  most  unfortunate  for  the  credit  of  American  astron- 
omers, as  well  as  for  the  good  of  the  science,  that  these  col- 
lections are  not  yet  suitably  published. 

One  would  think  that  the  100,000  stars  of  Bessel  and 
Argelander  would  have  been  sufficient  for  the  needs  of 
astronomy.  But  the  German  Astronomical  Society,  at  its 
meeting  in  Bonn,  in  1867,  deliberately  resolved  upon  the 
task  of  accurately  determining  the  position  of  every  star  as 
bright  as  the  ninth  magnitude  contained  in  Argelander's 
"  Durchmusterung." 

The  veteran  Argelander  presided  at  this  meeting,  and  it 
is  interesting  to  note  how  serious  the  undertaking  appeared 
to  be  to  him.  No  one  knew  better  how  gigantic  a  task  it 
was.  The  plan  was  well  laid.  A  set  of  539  very-well-deter- 
mined stars  was  assumed  as  fundamental,  and  the  society 
resolved  that  the  position  of  the  stars  to  be  determined 
should  be  referred  to  these.  The  sky  was  cut  up  into  zones 
five  degrees  wide,  and  various  observatories  undertook  to 
finish  one  or  more  of  these  zones.  The  Polar  Zone  (90° 
to  80°  north  of  the  equator)  had  lately  been  completed  by 
Carrington,  in  England,  and  did  not  need  revision. 

The  observatories  of  Kazan  (8o°-75°),  Dorpat  (75°- 
70°),  Christiania  (7o°-65°),  Helsingfors  (65°-55°),  Har- 
vard University  (55°-5O°),  Bonn  (5o°-4O°),  Lund  (40°- 
35°),  Leyden  (35°-3O°),  Cambridge,  England  (3O°-25°), 
Berlin  (25°-i5°),  Leipsic  (i5°-5°),  Albany  (5°-!°),  Niko- 
laief  (i°-2°  south),  joined  in  the  work,  and  to-day  it  is 
nearly  completed. 

But  this  is  only  a  beginning.  Schoenfeld's  "  Durch- 
musterung "  to  23°  south  will  soon  be  printed,  and  it  is 


SIDEREAL  ASTRONOMY:    OLD   AND  NEW          269 

the  intention  of  the  German  Astronomical  Society  to  push 
the  zones  to  this  point,  to  join  on  to  the  great  series  of 
southern  zones  printed  by  our  countryman,  Dr.  B.  A. 
Gould,  at  the  National  Observatory  of  the  Argentine  Re- 
public. Dr.  Gould  is  himself  a  pupil  of  Argelander,  and 
his  magnificent  work  may  be  fairly  called  an  outcome  of 
the  spirit  of  Bessel,  the  master;  105,000  observations  of 
some  73,000  stars,  from  23°  south  to  65°  south  of  the 
equator,  have  been  printed  by  Dr.  Gould  as  part  of  the  re- 
sults of  fourteen  years'  labour  in  a  foreign  country.  Thus 
from  the  north  to  the  south  poles  the  labours  of  Carrington, 
Argelander,  Bessel,  Gould,  and  Gilliss3  have  given  us  an 
almost  complete  catalogue  of  accurate  positions  of  nearly  all 
the  principal  stars.  Besides  this  we  shall  shortly  have  the 
region  from  80°  north  to  2°  south  completely  reobserved, 
and  by  1900  the  region  to  23°  south  will  be  done  also. 

SPECIAL  CATALOGUES  OF  STARS 

Besides  these  gigantic  undertakings  there  have  been 
scores  of  separate  catalogues  pretending  to  greater  pre- 
cision even,  the  very  names  of  which  we  can  not  mention. 
The  observatories  of  Greenwich,  Oxford,  Edinburgh,  Paris, 
Pultowa,  Dorpat,  Bonn,  Berlin,  Palermo,  Washington, 
Harvard  University,  Melbourne,  Cape  of  Good  Hope,  and 
many  others  have  issued  such  accurate  collections. 

It  is  also  necessary  to  say  that  a  certain  small  number 
of  stars — several  thousands — have  had  their  positions  and 
motions  determined  with  extreme  precision;  and  of  these 
again,  a  few  hundreds  of  the  brightest  stars  have  been 
observed  for  so  long,  and  so  many  times,  that  their  result- 
ing positions  are  now  almost  as  accurate  as  they  can  be 
made,  and  their  motions  so  well  known  as  to  admit  of 
very  little  improvement  by  the  work  of  the  next  genera- 
tion. These  are  our  fundamental  stars,  so  called. 

Such,  then,  are  our  data:  a  few  hundred  stars  deter- 
mined with  the  last  degree  of  precision,  a  few  thousand 
nearly  as  well,  200,000  with  considerable  accuracy,  and 
"Two  Germans,  one  Englishman,  two  Americans. 


2/0  HOLDEN 

nearly  half  a  million  separate  stars  known  by  the  approxi- 
mate positions  of  the  "  Durchmusterungen,"  or  additional 
to  these  from  the  southern  zones.  We  can  add  to  these 
too  the  200,000  or  more  stars  laid  down  in  the  ecliptic 
charts  of  Paris,  Vienna,  and  Clinton  (New  York),  which 
serve  as  nets  to  catch  the  minor  planets  just  now,  but 
which  have  an  incalculable  value  as  accurate  pictures  of 
the  sky  at  a  given  instant. 

The  brightness  of  some  10,000  stars  is  very  accurately 
known,  and  that  of  nearly  half  a  million  has  been  very 
approximately  fixed.  Lastly,  the  distances  of  some  fifteen 
of  the  brighter  stars  from  the  earth  are  known  with  tolerable 
certainty,  and  that  of  a  few  more  with  a  good  degree  of 
approximation. 

These  are  the  materials  available— mighty  monuments 
to  human  ingenuity,  skill,  patience,  devotion.  But  what 
further  problems  will  they  solve  for  us?  What  far-reach- 
ing conclusions  can  be  drawn?  In  a  succeeding  article  I 
shall  try  to  show  to  what  results  a  combination  of  the  data 
so  painfully  accumulated  may  lead,  and  what  conclusions 
may  safely  be  drawn  even  now. 

The  science  of  the  positions  and  the  motions  of  the 
stars  is  not  so  young  as  that  other  science  so  well  described 
by  Professor  Langley  in  his  admirable  articles  on  "  The 
New  Astronomy  "  ("  The  Century  "  for  September,  Octo- 
ber, December,  1884,  and  March,  1885),  but  it  nas  its 
modern  period  as  well  as  the  historical  one  which  has  been 
here  set  forth.  The  old  astronomy  has  set  itself  to  solve 
such  problems  as  these:  What  is  the  rate  at  which  the 
whole  solar  system  is  moving  on  through  space?  What 
are  the  distances  and  what  are  the  masses  of  the  stars? 
What  is  the  shape  of  the  stellar  cluster  to  which  our  sun 
belongs?  Are  the  stars  in  general  broken  up  into  subordi- 
nate universes?  or  do  they,  as  a  whole,  form  one  mighty 
system,  with  one  common  motion? 

Some  of  these  and  other  such  questions  are  answered; 
some  seem  almost  unanswerable;  some  are  still  in  the  way 
of  solution. 


SIDEREAL  ASTRONOMY:    OLD  AND  NEW 


II.    THE   RESULTS  THAT   IT  HAS   ATTAINED 


271 


In  the  preceding  chapter  we  collected  the  data  which 
the  ancient  and  the  modern  astronomy  has  placed  at  our 
disposition.  We  saw  that  a  few  hundred  of  the  stars  have 
their  positions  fixed  with  the  last  degree  of  precision;  a 
few  thousand  are  known  nearly  as  well;  half  a  million  have 
their  places  approximately  known,  and  half  of  these  last  are 
tolerably  well  determined.  The  brightness  of  some  ten 
thousand  stars  is  well  known,  while  the  brightness  of  nearly 
half  a  million  is  known  with  fair  approximation.  The  dis- 
tances of  a  few  stars  (about  fifteen)  are  known  with  pre- 
cision; the  distances  of  a  few  more  are  approximately 
known. 

These  are  the  data  which  have  been  amassed  by  the  ob- 
serving astronomers  of  the  modern  period,  beginning  with 
Bradley  (1750).  In  the  present  paper  we  are  to  see  some 
of  the  general  conclusions  which  may  be  drawn  from  these 
data.  What  are  the  distances,  what  are  the  dimensions,  of 
the  stars?  What  is  the  orbit  in  which  our  sun,  with  its 
group  of  planets,  is  travelling?  What  stars  are  our  nearest 
neighbours  and  travelling  with  us?  Are  stars  in  general 
aggregated  into  systems  of  comparatively  small  size,  or  are 
the  stars  as  a  whole  collected  into  one  vast  system,  bound 
together  by  a  common  bond,  and  endowed  with  a  common 
motion? 

The  stellar  universe,  as  we  see  it  at  any  moment,  is 
quite  complete.  Change  does  not  seem  to  belong  to  the 
region  of  fixed  stars.  Yet  every  one  of  the  millions  of 
observations  has  been  made  to  fix  a  position  so  accurately 
that  the  slow  changes  which  must  be  going  on  may  not 
escape  us;  so  that  the  laws  of  these  changes  can  be  for- 
mulated. If  we  know  that  a  star  retains  its  position  in- 
variably, if  we  know  positively  that  its  brightness  and 
colour  remain  the  same,  it  becomes  for  these  very  reasons 
a  most  useful  standard  of  reference,  but  it  does  not,  as  yet, 
help  us  to  solve  the  problem  of  the  stellar  universe.  We 
must  seek  a  clew  elsewhere,  among  the  stars  where  changes 


272 


HOLDEN 


are  manifest,  so  that  the  unknown  laws  of  these  changes 
may  be  unfolded. 

PROPER   MOTIONS  OF  STARS 

As  we  said,  nothing  appears  to  be  more  invariable  or 
unalterable  than  the  region  of  the  fixed  stars,  and,  in  a 
general  sense,  nothing  is  more  so.  But  when  we  come  to 
a  closer  view  all  is  change  there  as  well  as  elsewhere. 

Since  Rome  was  built  the  apparent  situation  of  Sirius 
has  changed  more  than  a  diameter  of  the  moon,  Arcturus 
has  moved  more  than  three  such  angular  diameters,  and 
so  with  other  stars. 

If  gravitation  is  truly  universal,  if  all  the  stars  are 
bound  together  in  one  system  by  this  law,  as  we  believe, 
then  no  star  can  move  without  affecting  every  other.  As 
one  moves  all  must  move.  The  real  motion  of  any  star 
is  along  some  line  or  curve;  we  see  this  real  motion  pro- 
jected on  the  ground  of  the  heavens  as  an  apparent  change 
of  its  latitude  and  longitude.  Knowing  the  latitude  and 
longitude  of  the  star  now,  by  observation,  we  may  com- 
pare these  with  the  positions  of  twenty,  fifty,  or  a  hundred 
years  ago.  It  is  possible  to  allow  by  calculation  for  every 
one  of  the  complex  changes  produced  in  the  apparent  posi- 
tion of  a  star  by  every  cause  not  in  the  star  itself.  Each 
one  of  the  several  observations,  when  reduced  to  a  com- 
mon epoch,  should  give  the  same  position,  except  for  the 
small  and  unavoidable  errors  of  observation  and  the  proper 
motion  of  the  stars. 

For  example,  here  are  the  observations  made  by  Dr. 
Gould  in  the  last  twelve  years  on  a  southern  star,  all  re- 
duced to  what  they  would  have  been  if  all  had  been  made 
on  January  i,  1875: 


YEAR  OF  OBSERVATION. 

Right  ascension. 

South  declination. 

1877  .  , 

23^  58m  O8   Q2 

a*7°  eg'  TI*  o 

1876  

2IO 

jl     0°    *-5  -V 

1881  

46^ 

>IA         T 

1885... 

6    60 

4.2    o 

SIDEREAL   ASTRONOMY:    OLD   AND  NEW          273 

These  do  not  agree.  They  ought  not  to  differ  by  more 
than  o?.2O  or  3"  4  if  the  star  were  at  rest.  If  we  assume 
that  the  star  is  moving  in  right  ascension  by  os-482  and 
in  declination  by  2". 45  yearly,  and  apply  these  numbers, 
the  positions  will  harmonize. 

1873  is  two  years  before  1875,  and  we  add  twice  os-482 
and  twice  2". 45;  and  subtract  for  the  other  intervals.  The 
observations  thus  corrected  give 

For  1873 23h  58mi«.88 37*  58'  i8".8 

1876 1.71 18.4 

1881 1.74 19.4 

1885 1.78 17.5 

and  are  harmonious  within  the  errors  of  observation.  If 
we  assume  that  this  star  is  as  near  to  the  earth  as  the  very 
nearest  of  all  the  stars,  it  is  certainly  moving  no  less  than 
600,000,000  miles  per  year.  Yet  it  will  require  more  than 
3,000  years  for  it  to  move  from  its  present  place  by  so 
much  as  one  diameter  of  the  moon. 

The  calculation  that  has  been  outlined  here  for  one 
star  has  been  performed  for  several  thousands  of  the  better- 
known  stars,  especially  for  the  3,222  stars  which  were  most 
carefully  determined  by  Bradley  in  1750.  For  each  one  of 
these  the  proper  motion  has  been  determined  with  the 
greatest  nicety.  The  results  at  first  sight  are  interesting 
only  in  a  very  special  way.  No.  i,  for  example,  may  be 
moving  21"  in  a  century  along  a  path  inclined  by  10°  to 
the  equator.  No.  2  moves  44"  in  a  century  along  another 
path  inclined  by  another  angle,  and  so  on  to  No.  3,222. 
Here  seem  to  be  three  thousand  isolated  facts,  each  one 
useful  in  its  narrow  relations,  but  each  having  no  connec- 
tion with  any  other. 

Let  us  suppose  for  a  moment  that  the  sun,  with  the 
solar  system,  and  the  earth,  our  point  of  view,  are  moving 
onward  in  space,  and  imagine  how  such  a  motion  would 
affect  the  appearance  of  a  universe  of  stars  scattered  all 
about  us.    If  the  sun  alone  has  a  motion,  all  the  stars  to- 
*  Errors  of  observation  of  this  magnitude  may  exist 
18 


274  HOLDEN 

ward  which  we  are  moving  will  appear  to  be  retreating  en 
masse  from  the  point  in  the  sky  toward  which  our  course 
is  directed.  The  nearer  stars  will  move  most  rapidly;  those 
more  distant,  less  so. 

In  the  same  way  the  stars  from  which  we  are  retreat- 
ing will  appear  to  crowd  together  and  approach  each  other. 
It  is  as  if  one  were  riding  on  the  rear  of  a  railroad  train 
and  watching  the  rails  over  which  one  had  just  passed.  As 
one  recedes  from  any  point  the  rails  at  that  point  seem  to 
come  nearer  and  nearer  together.  If  we  were  passing 
through  a  forest  we  should  see  the  trunks  of  the  trees  from 
which  we  were  going  apparently  moving  nearer  and  nearer 
to  each  other,  while  those  at  the  sides  would  retain  their 
distance  apart  and  those  in  front  would  be  moving  wider 
and  wider  apart. 

Here  is  a  case  in  which  we  are  sensible  of  our  own 
motion  and  observe  the  effects  of  that  motion  in  the  posi- 
tions of  the  fixed  objects  about  us.  We  may  turn  the  ques- 
tion about,  and  inquire  whether  the  observed  motions  of 
the  stars  indicate  any  real  motion  of  our  own. 

The  outline  of  the  problem  is  here  much  as  it  pre- 
sented itself  to  Sir  William  Herschel  in  1782.  The  de- 
tails are  extremely  complicated.  It  is  certain  that  we 
are  not  passing  along  through  space  among  a  vast  number 
of  fixed  stars.  Each  star  has  a  motion  peculiar  to  itself.  It 
also  is  moving  along  a  vast  orbit,  and  this  real  motion  of 
the  star  is  evident  to  our  instruments.  Combined  with  the 
veritable  motion  of  the  star  itself  is  the  parallactic  motion 
produced  by  the  shifting  of  our  own  point  of  view  as  the 
earth  sweeps  forward  through  space. 

It  is  for  analysis  to  separate  the  effects  of  these  two 
motions  and  to  determine  what  is  the  real  direction  and  the 
real  amount  of  the  solar  motion.  The  processes  of  the 
analysis  can  not  be  given  here,  but  fortunately  it  is  easy 
to  exhibit  both  the  data  and  the  results  graphically.  This 
has  been  well  done  by  M.  Flammarion  in  his  edition  of 
Dien's  Star-Atlas. 

The  circle  marked  "  Northern  Hemisphere  "  gives  the 


SIDEREAL  ASTRONOMY:    OLD  AND   NEW          275 

positions  of  those  northern  stars  which  are  known  to  have 
a  proper  motion.  The  size  of  the  dot  representing  each 
star  gives  the  magnitude  (i.  e.,  brilliancy)  of  the  star.  The 
arrows  attached  to  the  star  represent  the  directions  in  which 
the  stars  move  on  the  surface  of  the  sky  by  their  proper 
motions.  The  lengths  of  the  arrows  represent  the  velocities 
with  which  the  stars  move.  At  the  time  of  making  the 
map  the  stars  are  in  the  positions  marked  by  the  dots.  At 
the  end  of  50,000  years  they  will  be  at  the  ends  of  their 
respective  arrows. 

Thus  the  data  are  all  presented  graphically.  Notice 
what  variety  there  is.  Notice,  too,  the  striking  fact  that 
some  of  the  largest  proper  motions  belong  to  some  of  the 
smallest  stars.  One  would  think  that  the  brighter  stars 
would  be  the  nearer,  and  therefore  that  on  the  average  they 
would  have  the  larger  proper  motions.  For  evidence  on 
this  point  I  have  compiled  the  paragraph  which  follows 
from  Argelander's  list  of  the  250  stars  with  the  best-known 
proper  motions.  I  have  chosen  the  fainter  magnitude 
classes  in  order  to  get  a  sufficient  number  of  stars: 

"  Seventy-seven  stars  between  sixth  and  seventh  mag- 
nitudes have  a  proper  motion  of  o".54  yearly;  80  stars  be- 
tween seventh  and  eighth  magnitudes  have  a  proper  motion 
of  o".$6  yearly;  58  stars  between  eighth  and  ninth  magni- 
tudes have  a  proper  motion  of  o".?!  yearly." 

That  is,  the  proper  motions  do  not  seem  to  diminish 
as  the  numerical  magnitude  diminishes. 

But  to  return  to  the  plate.  In  the  constellation  Her- 
cules, not  far  from  the  bright  star  Vega,  which  is  near  our 
zenith  in  the  summer  sky,  is  the  point  toward  which  the 
sun  is  moving.  In  the  corresponding  position  on  the  map 
of  the  southern  hemisphere  is  a  similar  point;  it  is  the 
point  from  which  we  come.  All  over  the  map  are  arrows 
not  attached  to  any  stars.  These  show  the  direction  and 
the  velocity  of  that  part  of  the  proper  motion  due  to  the 
motion  of  the  solar  system  alone.  In  general  the  arrows 
belonging  to  the  stars  should  agree  in  length  and  in  direc- 
tion with  these  unattached  arrows — and  in  general  they 


276  HOLDEN 

do,  for  the  latter  were  derived  from  computations  based 
on  the  former.  But  there  are  many  exceptional  cases; 
and,  at  first  glance,  it  is  the  exceptions  which  seem  to  be 
the  rule. 

There  is  no  space  to  refer  to  special  cases  except  in 
passing;  but  we  should  note  a  pair  of  stars  marked  21,258 
(of  Lalande's  catalogue)  and  1,830  (of  Groombridge's 
catalogue).  They  were  about  15°  apart  in  1880.  In 
50,000  years  they  will  be  more  than  200  diameters  of  the 
moon  apart,  while  now  they  are  not  more  than  30  such  an- 
gular diameters.  Proper  motion  alone  will  in  time  change 
the  whole  aspect  of  the  sky. 

So  much  for  the  map.  Mathematical  analysis  gives 
the  same  results  in  numbers.  It  declares  that  the  apex  of 
solar  motion  is  in  right  ascension  260°  and  in  declination 
36°  north,  which  defines  the  point  in  Flammarion's  map 
marked  by  the  figure  like  the  sun;  and  analysis  further  de- 
clares that  the  amount  of  the  solar  motion  in  100  years,  if 
viewed  from  a  point  at  the  average  distance  of  the  3,222 
Bradley  stars,  would  be  5°.O5. 

If  we  know  this  average  distance  in  miles,  we  can  assign 
our  own  velocity  in  miles.  With  our  best  present  knowl- 
edge, it  follows  that  the  sun,  the  earth,  and  the  whole  solar 
system  are  moving  through  space  at  the  rate  of 

586,000,000  miles  per  year. 
1,600,000       "       "     day. 
67,000       "       "     hour. 
i8i     "       4<     second. 

The  earth  moves  about  the  sun  in  its  own  orbit  at  about 
the  same  rate  of  19  miles  per  second,  while  sun,  earth,  and 
orbit  move  along  in  space  another  19  miles. 

We  can  now  go  back  to  the  stars  themselves,  and  sub- 
tract from  the  observed  proper  motion  of  each  star  that 
portion  (motus  parallacticus)  which  is  due  to  the  motion  of 
the  solar  system,  and  leave  that  portion  which  is  due  to  the 
star's  own  motion  (motus  peculiaris). 


SIDEREAL   ASTRONOMY:    OLD   AND   NEW 


277 


Is  there  anything  common  to  the  truly  proper  motions 
of  the  stars?  In  the  first  place,  it  may  be  said  that,  so  far 
as  we  know  up  to  this  time,  these  motions  are,  in  general, 
not  curved.  They  are  practically  straight  lines.  They  have 
no  common  centre.  There  is  no  great  central  body  around 
which  revolve  the  suns  of  all  other  systems.  If  there  be 
such  a  body  it  will  be  many  centuries  before  we  shall  know 
it;  and  we  may  certainly  say  that,  so  far  as  our  knowledge 
goes,  there  is  none. 

SYSTEMATIC  MOTIONS  OF  THE  FIXED  STARS  PARALLEL  TO  THE 

MILKY    WAY 

But  if  we  are  obliged  to  consider  the  motions  of  all  the 
stars  to  be  practically  in  right  lines,  and  not  in  closed  orbits, 
there  is  no  reason  why  we  should  not  examine  the  ques- 
tion of  whether  the  stars  as  a  whole  do  not  have  some 
systematic  motion — whether  there  is  not  among  this  variety 
some  unity.  The  most  natural  hypothesis  to  start  with  is 
that  the  stars  have  a  vast  rotation  in  planes  parallel  to  the 
Milky  Way.  We  already  have  good  data  for  examining 
this,  and  in  a  few  years,  when  the  zones  of  the  "  Astrono- 
mische  Gesellschaft  "  are  complete,  much  material  will  be 
added.  Without  some  assumption  of  the  sort,  that  the  stars 
rotate  in  planes  parallel  to  the  Milky  Way,  it  is  hardly  pos- 
sible to  explain  the  existence  of  the  Milky  Way  itself.  It 
would  necessarily  disintegrate  more  and  more  with  the 
lapse  of  time,  and  it  would  be  a  pure  accident  that  we  hap- 
pen to  live  at  an  epoch  when  this  disintegration  has  not 
been  accomplished.  The  investigation  of  this  possible  rota- 
tion has  been  carried  out  by  two  pupils  of  Professor  Gylden 
and  of  Professor  Schoenfeld  respectively.  While  the  result 
in  one  case  is  fairly  against  the  hypothesis  of  such  a  rota- 
tion, in  the  other  it  is  somewhat  in  its  favour.  The  doubt  in 
the  matter  arises  solely  from  the  deficiency  of  the  data,  and 
this  will  soon  be  supplied.  In  the  mean  time  it  should  be  an 
answer  to  those  objectors  who  ask  what  is  the  use  of  another 
new  catalogue  of  stars,  that  this  catalogue,  and  every 
other  catalogue,  goes  a  certain  way  toward  providing  the 


278  HOLDEN 

means  for  solving  the  very  greatest  problem  that  can  be 
presented  to  the  human  mind  by  natural  objects. 

Look  at  the  Milky  Way  stretching  across  the  summer 
sky  with  the  bright  star  Vega  burning  near  it.  Think  that 
the  few  proper  motions  laboriously  determined  by  Halley 
and  Maskelyne  enabled  Herschel  to  announce  that  the  sun, 
the  earth,  and  every  planet  is  moving  toward  a  spot — near 
Vega — which  he  could  point  out.  Think,  too,  that  the 
smallest  efforts  of  every  faithful  observer,  the  world  over, 
go  to  the  solution  of  the  question,  How  do  all  these  thou- 
sands of  stars  that  I  see  move  in  space?  Are  they  bound 
up  with  that  Milky  Way  in  one  fate?  Or  is  that  permanent 
shining  track,  which  seems  unchanged  since  Job  and  the 
patriarchs  looked  upon  it — is  that  doomed  to  destruction? 
The  finger  of  analysis  can  point  out  the  fate  of  those  myri- 
ads of  shining  stars,  and  man  becomes  fit  to  live  under 
their  influence  when  his  mind  adds  the  beauty  of  law  to 
the  wayward  beauty  of  their  shining. 

SPECTROSCOPIC   PROPER   MOTIONS — MOTIONS   IN   THE    LINE   OF 

SIGHT 

The  observation  of  a  star's  position  is  really  nothing  but 
the  determination  of  the  place  where  the  line  joining  eye 
and  star  pierces  the  celestial  sphere.  The  determination  of 
its  proper  motion  is  nothing  but  the  determination  of  the 
rate  at  which  its  apparent  position  changes.  If  a  star  is 
moving  directly  toward  us,  or  directly  away  from  us,  its 
apparent  place  in  the  sky  will  remain  unchanged.  But  we 
have  in  the  spectroscope  a  means  of  measuring  the  motion 
of  a  star  in  the  line  of  sight.  The  principle  of  the  method 
is  simple.  The  application  of  it  is  most  difficult.  Every 
one  has  noticed,  in  travelling  upon  an  express  train,  the 
sudden  clang  of  the  bell  of  a  train  passing  in  the  contrary 
direction;  and  how  the  note,  the  pitch,  of  the  sound  of 
this  bell  rapidly  changes  from  high  back  to  low  again. 
Nothing  is  more  certain  than  that  the  bell  has  but  one 
essential  pitch.  Why,  then,  does  it  change?  The  engineer 
of  the  passing  train  hears  his  own  bell  giving  always  the 


SIDEREAL  ASTRONOMY:    OLD  AND  NEW          279 

same  note,  and  this  note  is  determined  by  the  length  of  the 
sound  waves  that  reach  his  ear.  Suppose  them  to  come  at 
the  rate  of  about  500  per  second  to  him.  He  is  always 
moving  at  the  same  rate  as  his  bell.  But  to  us  in  the  other 
train  the  case  is  different.  When  the  bell  is  just  opposite 
us,  500  waves  come  to  us  per  second;  when  we  are  ap- 
proaching the  passing  train  more  than  500  come  to  us  (not 
only  the  500  sent  out  by  the  Bell,  but  those  others  which 
we  meet  by  our  velocity);  as  we  leave  the  passing  train 
less  than  500  waves  overtake  us  per  second.  Hence  the 
pitch  (the  number  of  waves  per  second)  varies.  The  same 
thing  happens  in  the  case  of  light.  In  the  spectrum  of  a 
star  there  are  certain  dark  lines  the  presence  of  which  is 
due  to  hydrogen  in  the  star's  atmosphere.  If  the  star  is 
at  rest  with  respect  to  us,  these  lines  are  not  displaced  in 
its  spectrum;  a  definite  number  of  waves  per  second  (say 
A)  come  to  us  from  the  spectrum  on  both  sides  of  these 
lines.  If  the  star  is  approaching  us,  more  waves  than  A 
reach  us;  if  the  star  is  receding,  fewer  waves  reach  us.  The 
pitch  of  the  line,  so  to  say,  is  altered;  and  the  spectroscope 
can  measure  this  change  of  pitch. 

When  this  is  done  with  respect  to  the  principal  stars 
the  most  interesting  results  follow. 

Vega  (a  Lyrse)  is  found  to  be  approaching  us  at  the  rate 
of  10  miles  per  second.  Castor  is  approaching  us  at  18 
miles,  Arcturus  at  5  miles,  etc.;  while  Pollux  is  receding 
from  us  i  mile  per  second,  Aldebaran  is  receding  30  miles, 
and  so  on.  After  years  the  aspect  of  our  sky  will  change. 
We  shall  have  new  glories  in  the  galaxy,  and  after  thou- 
sands of  years  these  again  will  leave  us.  There  is  ceaseless 
change  here  as  everywhere.5 

PARALLAXES    OF  THE   STARS 

The  ancients  placed  all  the  fixed  stars  on  the  inner  sur- 
face of  a  vast  sphere  which  turned  about  the  earth's  centre 

8  Later  observations,  particularly  those  at  Potsdam  and  at  the  Lick 
Observatory,  have  given  more  accurate  velocities  than  those  originally 
printed,  and  the  later  values  have  been  inserted  here. 


280  HOLDEN 

once  each  day.  They  had  absolutely  no  way  of  even  guess- 
ing how  far  off  this  sphere  might  be.  In  1618  Kepler's 
guess  was  4,000,000  times  as  far  as  the  sun;  in  1698  Huy- 
gens  placed  Sirius  28,000  times  as  far  as  the  sun;  in  1841 
Picard  showed  that  the  errors  of  observation  with  the  in- 
struments of  his  time  were  as  great  as  the  parallaxes  of 
the  stars  themselves,  and  that  therefore  the  problem  was 
indeterminate  to  him;  in  1806  Delambre  concluded  that 
the  same  thing  remained  true,  notwithstanding  the  im- 
provements of  the  instruments  in  the  meanwhile.  It  was 
not  till  1836  that  W.  Struve  and  Bessel  really  determined 
the  parallax,  and  hence  the  distance,  of  two  different  stars 
a  Lyrae  and  61  Cygni. 

It  is  familiar  to  all  that  the  distances  of  even  the  near- 
est stars  are  not  to  be  conceived  when  they  are  expressed 
in  miles  or  familiar  units.  No  star  is  so  near  to  us  as 
200,000  times  93,000,000  miles.  We  have  to  express 
these  distances  in  terms  of  the  time  required  for  light  to 
pass  from  star  to  earth.  For  61  Cygni  that  time  is  2,377 
days,  or  6J  years.  It  was  the  elder  Herschel  who  put  these 
immense  distances  before  us  in  the  true  light,  by  showing 
that  if  to-day  the  star  were  blotted  out  of  existence  its  mild 
light  would  shine  on  for  years,  until  the  last  ray  that  left 
it  had  finally  ended  its  long  journey  and  reached  the  earth, 
more  than  six  years  afterward. 

But  all  stars  are  not  equally  distant.  The  light  from 
one  star  may  be  10,  from  another  100,  from  another  1,000 
years  old  when  it  reaches  us.  We  must  no  longer  regard 
the  study  of  the  stars  as  a  study  of  their  contemporaneous 
existence.  It  is  rather  the  ancient  history  of  the  universe 
which  is  exhibited  to  us  by  the  vault  of  heaven.  Assiduous 
observers  have  determined  the  parallaxes  of  about  a  score 
of  stars.  The  first  stars  to  be  examined  were  either  the 
brightest  (as  in  the  case  of  Vega),  or  those  of  large  proper 
motion  (as  61  Cygni).  In  general,  the  brightest  stars  should 
be  the  nearest,  one  would  think,  and  yet  the  very  largest 
parallaxes  belong  to  the  fainter  stars.  Similarly  the  star 
with  the  greatest  proper  motion  has  a  very  small  parallax. 


SIDEREAL  ASTRONOMY:    OLD  AND  NEW          28 1 

By  treating  all  the  certain  data  in  various  ways,  Pro- 
fessor Gylden  has  come  to  the  conclusion  that  the  average 
parallax  of  a  star  of  the  first  magnitude  is  about  o".o84,  or 
that  the  average  distance  of  our  brightest  star  is  160,000,- 
000,000,000,000  miles.  But  to  make  further  steps  in  the 
problem  of  the  "  construction  of  the  heavens,"  we  must 
know  more  than  the  average  parallax  of  the  brightest  stars. 
We  must  be  able  to  assign  the  average  parallax  of  stars  of 
each  order  of  magnitude,  and  this  in  both  hemispheres. 

This  task  is  now  undertaken  for  stars  down  to  the 
fourth  magnitude  by  two  observers  who  have  already  dis- 
tinguished themselves  in  this  field — Dr.  Gill,  Royal  Astron- 
omer at  the  Cape  of  Good  Hope,  and  Dr.  Elkin,  now  at 
Yale  University  Observatory.  These  gentlemen  have  de- 
voted their  energies  to  this  one  problem,  which  will  require 
perhaps  ten  years  for  its  solution  in  the  form  that  they  have 
chosen  for  it.  Dr.  Ball,  Royal  Astronomer  for  Ireland,  is 
systematically  searching  for  stars  of  large  parallax  and  in- 
cidentally proving  many  stars  to  have  small  parallax — a 
fact  which  it  is  just  as  important  to  know  as  its  converse. 

The  next  dozen  years  will  show  immense  strides  in  our 
knowledge  of  the  stellar  distances  of  individual  stars,  and 
it  may  well  be  that  some  general  relation  between  distance, 
brightness,  and  proper  motion  of  situation  in  the  sky  will 
result  from  the  great  increase  of  data. 

DISTANCES   OF  STARS  OF  EACH   MAGNITUDE 

The  golden  time  for  astronomers  will  come  when  the 
parallaxes  of  enough  stars  have  been  determined  for  them 
to  be  able  to  say  that  the  distance  of  an  average  third, 
fourth,  sixth,  or  tenth  magnitude  star  is  so  many,  or  so 
many,  times  the  sun's  distance.  That  time  has  not  yet 
come,  nor  will  it  have  come  even  when  the  great  work 
begun  by  Messrs.  Gill  and  Elkin  has  been  ended.  There 
is  no  certain  way  of  assigning  the  stellar  distances  but  by 
measurements  such  as  they  are  making.  But  it  is  a  fair 
procedure  to  make  certain  assumptions  as  to  stellar  dis- 
tances, to  work  out  the  logical  consequences  of  these  as- 


282  HOLDEN 

sumptions,  and  to  compare  these  consequences  with  known 
facts.  An  agreement  with  the  facts  will,  in  some  degree, 
support  the  assumptions.  If  we  assume  the  stars  to  be  of 
equal  brilliancy  one  with  another,  we  have  one  basis  of 
computation.  If  we  suppose  them,  further,  to  be  equally 
distributed  in  space  on  the  average,  we  have  another  basis. 
These  conditions  lead  at  once  to  the  following  table: 

Magnitudes.  Relative  distances. 

I ...       1. 00 

2 1.54 

3 2.36 

4 3-64 

5 5-59 

6 8.61 

7 13-23 

8 20.35 

We  can  test  these  assumptions  to  some  extent.  If  they 
are  true,  then  the  ratio  of  the  actual  number  of  stars  of 
any  brightness  to  the  actual  number  of  stars  of  the  next 
lower  grade  of  brightness,  raised  to  the  two-thirds  power, 
should  be  0.400.  Using  the  stars  of  the  sixth  and  seventh 
magnitudes,  this  number  results  0.426;  of  the  seventh  and 
eighth,  it  results  0.4003,  etc.  The  two  hypotheses  are  in 
the  main  not  far  from  correct,  and  therefore  the  relative  dis- 
tances above  given  are  not  very  far  wrong  for  stars  down 
to  the  eighth  magnitude.  There  is  strong  reason  to  believe 
that  the  fainter  stars,  from  eleventh  to  fifteenth  magnitudes, 
do  not  follow  the  same  law.  We  have  seen  that  the  average 
distance  of  a  first-magnitude  star  is  160,000,000,000,000,- 
ooo  miles.  Multiply  this  by  20.35  an^  you  have  the  best 
estimate  now  available  of  the  distance  of  an  eighth-magni- 
tude star.  It  is  inconceivable,  but  no  more  so  than  the  first 
number.  Light  would  require  600  years  and  more  to  reach 
us  from  such  stars. 

DISTRIBUTION   OF   THE    STARS    OVER   THE    SURFACE    OF   THE 
CELESTIAL    SPHERE 

The  real  question  to  be  solved  is,  How  are  the  stars  dis- 
tributed throughout  solid  space  itself?  To  solve  this  ques- 
tion completely  the  distance  of  every  star  from  the  earth 


SIDEREAL  ASTRONOMY:    OLD  AND  NEW          283 

must  be  measured  (which  is  a  simple  impossibility),  or  else 
we  must  find  some  law  which  connects  the  brightness,  or  the 
proper  motion,  or  the  position  of  a  star  with  its  distance. 
Suppose  that  ten  stars  of  each  magnitude  from  the  bright- 
est down  to  the  faintest  are  selected — say  150  or  160  in  all 
— and  that  the  parallax  of  each  individual  star  is  determined. 
This  would  be  a  tremendous  labour  in  itself,  and  would 
require  the  work  of  several  observers  for  a  score  of  years. 
But  suppose  this  work  done.  Suppose  that  the  average 
distances  of  the  ten  stars  of  each  group  resulting  from 

the  measures  were  I,  II,  III,  IV,  V XIII, 

XIV,  XV,  XVI.  Would  any  general  relation  exist  be- 
tween the  magnitudes  I 16  and  the  correspond- 
ing distances  I XVI  ?  From  the  measures  that 

we  already  possess  this  is  by  no  means  sure.  In  fact, 
the  evidence  seems  to  be  directly  opposed  to  this  conclu- 
sion. The  average  measured  parallax  of  five  first-magni- 
tude stars  is  about  o".27;  of  three  fourth-magnitude  stars 
about  o".i3;  of  three  fifth-magnitude  stars  about  o".3i; 
of  seven  sixth-magnitude  stars  about  o".2i.  That  is, 
the  parallax  does  not  seem  materially  to  decrease  as  the 
brilliancy  diminishes  from  the  first  to  the  sixth  magnitude. 
If,  instead  of  comparing  the  magnitudes  with  the  distances, 
we  compare  the  proper  motions,  there  seems  to  be  no  evi- 
dent agreement.  The  stars  with  the  largest  proper  motions 
do  not  in  general  have  the  largest  parallaxes  (and  hence 
the  smallest  distances).  We  have  not  enough  determina- 
tions of  parallax  to  decide  whether  the  region  of  the  sky 
in  which  a  star  is  situated  has  any  relation  to  its  distance; 
so  that  for  the  present  we  are  not  sure  that  a  series  of 
measures  even  so  extensive  as  the  one  we  have  imagined 
would  solve  the  question  of  the  relation  between  magni- 
tude, or  proper  motion,  and  parallax.  Such  a  series  would 
go  a  great  way  toward  deciding  whether  the  question  was 
solvable  or  not.  It  would  add  enormously  to  the  very 
small  number  of  certain  facts  bearing  on  the  subject  of  the 
constitution  of  the  stellar  system.  And  it  is  to  the  great 
credit  of  this  generation  of  astronomers  that  such  a  series 


284  HOLDEN 

has  actually  been  begun  (for  stars  of  from  first  to  fourth 
magnitudes)  by  Messrs.  Gill  and  Elkin  at  the  Cape  of  Good 
Hope  and  New  Haven  respectively,  as  has  been  mentioned 
already. 

In  the  absence  of  definite  knowledge  with  regard  to  the 
distribution  of  the  stars  in  space,  much  labour  has  been  ex- 
pended on  the  study  of  what  we  may  call  stellar  statistics — 
the  statistics  of  the  distribution  of  the  stars  on  the  surface 
of  the  celestial  vault.  This  distribution  of  the  stars  is  known 
when  once  we  have  a  map  of  their  positions,  which  it  is 
comparatively  easy  to  make.  A  more  rapid  method  of 
studying  this  distribution  may  be  employed — that  of  star 
gauging,  so  called  by  Herschel,  its  inventor.  This  con- 
sists essentially  in  counting  the  number  of  stars  visible  in 
the  field  of  the  telescope  as  it  is  directed  to  various  known 
portions  of  the  sky.  The  mere  number  of  stars  visible  at 
each  pointing  may  be  laid  down  on  a  map,  like  the  sound- 
ings on  a  hydrographic  chart.  The  data  are  easily  gath- 
ered. How  are  they  to  be  interpreted?  We  may  briefly 
indicate  one  obvious  method.  Suppose  that  we  have  made 
such  star  gauges  with  telescopes  of  five  different  powers 
over  the  same  areas  in  the  sky.  The  largest  telescope  will 
show  all  the  stars,  say,  down  to  and  including  the  fifteenth 
magnitude;  the  next  smaller  those  to  the  fourteenth;  the 
next  to  the  thirteenth,  the  twelfth,  the  eleventh  (the  actual 
distribution  of  the  individual  stars  from  first  to  tenth  mag- 
nitudes is  known  by  the  "  Durchmusterungen  ").  In  any 
area  the  difference  between  all  the  "  Durchmusterung  " 
stars  (from  one  to  tenth  magnitude)  and  the  number  seen 
in  telescope  I  (the  smallest  of  the  five  supposed)  will  give 
the  number  of  the  eleventh-magnitude  stars  in  that  region. 

The  difference  between  the  counts  by  telescope  I 
(which  shows  all  stars  down  to  and  including  the  eleventh 
magnitude)  and  telescope  II  (which  shows  all  to  twelfth 
magnitude)  will  give  the  actual  number  of  twelfth-magni- 
tude stars.  Combining  the  results  of  the  telescopes  II  and 
III  we  should  have  the  number  of  thirteenth-magnitude 
stars  for  this  region,  and  so  on  for  the  fourteenth  and 


SIDEREAL  ASTRONOMY:    OLD   AND   NEW          285 

fifteenth  magnitudes.  Thus  the  actual  number  of  the  stars 
of  each  magnitude  in  this  area  (and  similarly  for  other 
areas)  will  be  known.  We  may  interpret  these  figures 
somewhat  in  this  way:  Take  a  map  which  shall  have 
spaces  on  it  for  the  whole  sky,  and  devote  this  map  to  ex- 
hibiting the  results  of  our  gauges  for  the  fifteenth-magni- 
tude stars.  Wherever  there  are  100  of  these  to  the  square 
degree  lay  on  one  tint  of  colour;  wherever  there  are  200, 
two  tints;  300,  three  tints,  and  so  on.  The  final  map  will 
exhibit  to  the  eye  the  results  of  our  gauges  for  the  fif- 
teenth-magnitude stars.  Where  the  tint  is  deep,  there  are 
more  stars;  where  it  is  light,  fewer.  Another  such  map 
must  be  made  for  the  fourteenth-magnitude  stars;  another 
for  the  thirteenth,  and  so  on.  Now  place  these  fifteen  maps 
side  by  side  before  you,  and  it  will  be  possible  to  obtain 
at  once  a  number  of  definite  conclusions.  Here  the  stars 
that  we  call  fifteenth  and  those  that  we  call  fourteenth  are 
really  connected  together  in  space.  Why?  Because  this 
long  ray  of  many  fifteenth-magnitude  stars  on  one  map 
is  matched  by  this  other  long  ray  of  just  the  same  position 
and  shape  of  the  fourteenth-magnitude  stars.  The  thir- 
teenth, too,  we  will  say,  is  similar.  But  the  ninth,  tenth, 
eleventh,  and  twelfth  do  not  in  their  distribution  at  all  re- 
semble the  fainter  stars  in  this  region,  but  they  do  resemble 
each  other.  In  this  way,  passing  from  region  to  region, 
the  general  peculiarities  of  each  region  may  be  made  out, 
and  much  light  may  be  thrown  on  the  vital  question,  How 
many  magnitudes  of  stars  exist  at  the  same  distance  from 
us?  Are  the  stars  of  the  so-called  ninth,  tenth,  and  eleventh 
magnitudes  all  really  at  the  same  distance  from  us,  and  are 
their  differences  in  brightness  simply  due  to  differences  in 
size,  or  are  they  really  at  different  distances? 

A  large  amount  of  evidence  upon  these  fundamental 
points  already  exists,  and  more  is  being  accumulated,  and 
it  appears  possible  that  a  skilful  use  of  it  may  throw  much 
light  on  the  real  question.  The  new  photographic  pro- 
cesses will  be  of  immense  importance  for  this  investigation. 
We  have  not  the  space  to  go  further  into  this  method  of 


286  HOLDEN 

research,  but  we  may  just  refer  in  passing  to  one  interesting 
form  of  it.  We  have  already  elaborate  maps  of  certain 
portions  of  the  sky  showing  the  position  and  magnitude  of 
every  star  down  to  the  thirteenth.  These  are  the  maps 
used  for  the  discovery  of  asteroids.  From  each  of  these 
maps  we  can  make  thirteen  others,  each  one  of  which 
shall  show  the  stars  of  one  magnitude  only.  Now  compare 
these  thirteen  derived  maps,  and  see  what  the  evidence 
is  that  the  stars  of  any  two  magnitudes  are  connected  or 
independent.  This  method  is  capable  of  bringing  out  most 
interesting  conclusions  when  it  is  thoroughly  carried  out, 
as  it  has  not  yet  been  to  any  large  degree.  The  local  ar- 
rangements of  stars  can  be  adequately  studied  in  this  way, 
and  it  is  not  too  much  to  expect  that  the  typical  forms 
of  stellar  systems — distorted  by  perspective,  of  course — 
may  be  exhibited  here. 

MASSES   OF   BINARY  AND   OTHER  STARS 

The  binary  systems  are  those  composed  of  two  stars 
which  are  connected  with  each  other  by  a  mutual  gravita- 
tion. They  revolve  about  a  common  centre  of  gravity  in 
orbits  which  can  be  calculated.  In  some  few  cases  the 
parallax  of  these  stars  is  known;  and  in  every  such  case  the 
sum  of  the  masses  of  the  two  stars  becomes  known  in 
terms  of  the  mass  of  our  own  sun.  It  is  especially  note- 
worthy that  in  every  known  case  the  mass  of  the  binary 
system  is  not  very  different  from  the  mass  of  our  own  sun. 
That  is  to  say,  all  the  stars  whose  masses  are  known  at  all 
are  such  bodies  as  our  sun  is:  they  shine  with  light  like 
his;  they  are  of  the  same  order  of  magnitude  mass. 

The  term  "  hypothetical  parallax  "  is  applied  to  a  paral- 
lax computed  for  a  binary  star  on  the  supposition  that  the 
mass  of  the  binary,  although  unknown,  may  be  hypotheti- 
cally  assumed  to  be  the  same  as  the  sun's  mass.  So  far  as 
we  can  judge,  these  hypothetical  parallaxes  must  be  provi- 
sionally accepted  as  essentially  correct. 

If  we  can  assume  that  the  intrinsic  brilliancy  of  the 
fixed  stars  is  the  same  for  each  star,  which  does  not  seem 


SIDEREAL  ASTRONOMY:    OLD   AND   NEW          287 

to  be  a  very  violent  supposition,  several  interesting  con- 
clusions follow  which  can  only  be  stated  here. 

If  it  be  true  that  for  the  stars,  taken  one  with  another, 
a  square  mile  of  surface  shines  with  an  equal  light  for  each 
star,  then  among  stars  of  known  distances  some  must  be 
at  least  270  times  as  great  in  diameter  as  others.  This  is 
about  the  proportion  of  the  sun  to  Mercury.  Also  it  fol- 
lows that  binary  stars  whose  colours  are  alike  must  be  com- 
posed of  stars  of  like  size;  and  also,  that  on  the  average 
the  brightest  star  of  any  cluster  is  about  four  times  as  large 
as  the  smallest  star  of  the  cluster.  No  star  is  more  than 
200,000  times  farther  than  the  nearest  fixed  star.  Other 
assumptions  which  might  serve  as  a  basis  for  computation 
will  give  other  results;  but  for  the  present  we  have  to 
content  ourselves  with  some  such  assumption,  and  in  the 
infinite  variety  of  circumstances  choose  that  one  as  general 
which  seems  to  be  the  most  likely  a  priori,  and  which  leads 
to  results  which  agree  with  actual  observation. 

THE   CLUSTER   OF  STARS  TO  WHICH   OUR   SUN   BELONGS 

The  "  Uranometria  Nova "  of  Argelander  gave  the 
positions  of  the  lucid  stars  of  the  northern  sky,  and  it  has 
been  supplemented  by  the  "  Uranometria  Argentina  "  of 
Dr.  Gould,  which  covers  the  southern  sky.  With  the  stellar 
statistics  of  the  whole  sky  before  him  Dr.  Gould  was  in 
a  position  to  draw  some  extremely  interesting  conclusions 
with  respect  to  the  arrangement  of  the  brighter  stars  in 
space,  and  to  the  situation  of  our  solar  system  in  relation 
to  them.  The  outline  of  his  reasoning  can  be  given  here, 
but  the  numerical  evidence  upon  which  his  conclusions  are 
founded  must  be  omitted.  In  the  first  place,  it  is  assumed 
that  in  general  the  stars  that  are  visible  to  the  naked  eye 
(the  lucid  stars)  are  distributed  at  approximately  equal 
distances  one  from  another,  and  that  on  the  average  they 
are  of  approximately  equal  brilliancy.  If  we  make  a  table 
of  the  number  of  stars  of  each  separate  magnitude  in  the 
whole  sky  we  shall  find  that  there  are  proportionately  many 
more  of  the  brighter  ones  (from  first  to  fourth  magnitudes) 


288  HOLDEN 

than  of  the  fainter  (from  fourth  to  seventh  magnitudes) — 
that  is,  there  is  an  "  unfailing  and  systematic  excess  of  the 
observed  number  of  the  brighter  stars."  We  can  not  sup- 
pose, taking  one  star  with  another,  that  the  difference  be- 
tween their  apparent  brightness  arises  simply  from  real  dif- 
ference in  size,  but  we  must  conclude  that  the  stars  from 
the  first  to  fourth  magnitudes  (some  500)  are  really  nearer 
to  us  than  the  fainter  stars.  It  therefore  follows  that  these 
brighter  stars  form  a  system  whose  separation  from  that 
of  those  of  the  fainter  stars  is  marked  by  the  change  of 
relative  numerical  frequency. 

What,  then,  is  the  shape  of  this  system?  and  have  we 
any  independent  proof  of  its  existence?  Sir  John  Herschel 
and  Dr.  Gould  have  pointed  out  that  there  is  in  the  sky  a 
belt  of  brighter  stars  which  is  very  nearly  a  great  circle 
of  the  sphere.  This  belt  is  plainly  marked,  and  it  is  inclined 
about  80°  to  the  Milky  Way,  which  it  crosses  near  Cas- 
siopea  and  the  Southern  Cross.  Taking  all  the  stars  down 
to  4.0  magnitude,  Dr.  Gould  shows  that  they  are  more 
symmetrically  arranged  with  reference  to  this  belt  than 
they  are  with  reference  to  the  Milky  Way.  In  fact,  the 
belt  has  264  stars  on  one  side  of  it  and  263  on  the  other, 
while  the  corresponding  numbers  for  the  Milky  Way  are 
245  and  282.  From  this  and  other  reasons  it  is  concluded 
that  this  belt  contains  brighter  stars  because  it  contains 
the  nearest  stars,  and  that  this  set  of  nearer  and  brighter 
stars  is  distinctively  the  cluster  to  which  our  sun  belongs. 
Leaving  out  the  brighter  stars  which  may  be  accidentally 
projected  among  the  true  stars  belonging  to  this  cluster, 
Dr.  Gould  concludes  that  our  sun  belongs  to  a  cluster  of 
about  400  stars;  that  it  lies  in  the  principal  plane  of  the 
cluster  (since  the  belt  of  bright  stars  is  a  great,  not  a 
small  circle);  and  that  this  solar  cluster  is  independent  of 
the  vast  congeries  of  stars  which  we  call  the  Milky  Way. 

We  know  that  the  sun  is  moving  in  space.  It  becomes 
a  question  whether  this  motion  is  one  common  to  the  solar 
cluster  and  to  the  sun,  or  only  the  motion  of  the  sun  in 
the  solar  cluster.  The  motion  has  been  determined  on  the 


SIDEREAL  ASTRONOMY:    OLD  AND   NEW          289 

supposition  that  the  sun  is  moving  and  that  its  motion  is 
not  systematically  shared  by  the  stars  which  Dr.  Gould 
assigns  to  the  solar  cluster.  But  a  very  important  research 
will  be  to  investigate  the  solar  motion  without  employing 
these  400  stars  as  data. 

In  what  has  gone  before  I  have  tried  to  exhibit  some 
of  the  main  questions  in  purely  sidereal  astronomy;  to  show 
some  of  the  more  important  results  already  reached,  and 
especially  to  indicate  the  directions  along  which  present 
researches  are  tending.  It  is  impossible  to  give  a  complete 
view  in  this  or  in  any  other  single  branch  of  astronomy,  for 
they  are  all  indissolubly  bound  together. 

The  methods  of  the  new  astronomy  have  taught  us  that 
in  the  condition  of  the  variable  stars,  where  the  intense 
glow  has  cooled  to  a  red  heat,  we  can  see  the  future  of 
our  own  sun  as  well  as  its  past  in  the  brilliant  white  and 
violet  of  the  brightest  and  youngest  stars.  It  requires  the 
profound  mathematical  analysis  of  Gylden  to  interpret  his 
equations  so  as  to  explain  to  the  new  astronomy  exactly 
how  the  phenomena  of  the  rotation  of  variable  stars  pro- 
duce the  effects  which  are  observed  by  its  methods. 

Professor  Langley  measures  the  light  and  heat  of  the 
moon  by  the  new  methods;  Professor  Darwin  interprets 
the  mathematical  theory  of  the  tides  so  as  to  trace  back 
the  origin  of  that  heat  to  the  remote  time  when  the  earth 
and  moon  formed  one  mass,  and  rotated  in  less  than  an 
eighth  part  of  our  present  day.  All  the  parts  of  the  com- 
plex science  are  intimately  connected,  and  no  one  can  be 
separately  treated  without  losing  sight  of  many  lines  of 
research  of  the  greatest  promise  and  importance. 

But  I  hope  that  enough  has  been  said  to  show  that  the 
old  astronomy  is  not  idle;  that  it  has  its  new  side;  and  that 
its  energies  are  addressed  to  the  solution  of  problems  of  the 
highest  significance.  In  broad  terms,  it  is  the  noble  aim  of 
the  new  astronomy  to  trace  the  life-history  of  an  individual 
star,  and  of  the  old  to  show  how  all  these  single  stars  are 
bound  together  to  make  a  universe.  There  is  no  antago- 
nism in  their  objects.  Each  is  incomplete  without  the  other. 


PHOTOGRAPHY  THE   SERVANT   OF 
ASTRONOMY  J 

IN  order  to  appreciate  the  present  state  of  astronomy,  its 
new  methods,  its  novel  instruments,  its  recondite  prob- 
lems, it  is  necessary  to  glance  at  its  condition  a  half  cen- 
tury ago.  The  great  astronomers,  Bessel  and  W.  Struve, 
were  then  contending  in  friendly  rivalry  to  found  the  sci- 
ence on  a  sure  basis.  They  had  a  perfectly  definite  object, 
and  that  object  has  been  attained  through  their  efforts,  and 
through  the  efforts  of  the  school  of  young  men  whom  they 
trained  either  directly  or  indirectly — Argelander,  Schoen- 
feld,  Krueger,  Auwers,  Winnecke,  Wagner,  Schiaparelli  in 
Europe,  Walker,  Coffin,  Hubbard,  Gould  in  America. 

The  attention  of  astronomers  was  then  almost  exclu- 
sively directed  to  the  question  of  the  motions  of  the 
heavenly  bodies,  as  determined  by  the  law  of  universal 
gravitation.  The  vast  catalogues  of  stars  which  have  been 
made  in  the  past  half  century,  as  well  as  the  accurate  dis- 
cussion and  rediscussion  of  the  older  observations  of 
Bradley  (1750),  at  Greenwich,  were  all  undertaken  for  this 
sole  object.  The  school  of  mathematical  astronomers 
founded  by  Euler,  Laplace,  Lagrange,  Gauss,  utilized  these 
observations  to  the  utmost.  The  examination  of  the  sur- 
faces of  the  planets  was  an  entirely  secondary  question, 
and  was  largely  left  to  amateur  astronomers.  The  surface 
of  the  sun  was  studied  only  in  the  crudest  manner,  simply 
for  the  enumeration  of  the  solar  spots. 

The  fact  that  these  spots  were  periodic  was  only  estab- 
lished in  1851.  Sir  John  Herschel  was  almost  the  only 

1  From  the  "  Overland  Monthly,"  November,  1886. 
290 


PHOTOGRAPHY   THE   SERVANT   OF   ASTRONOMY 

astronomer  by  profession  who  devoted  himself  to  observa- 
tions not  "  of  precision." 

In  this  fifty  years,  an  entirely  new  science  has  arisen — 
astrophysics — which  is,  indeed,  the  daughter  of  astronomy, 
but  the  cousin-german  of  chemistry,  technics,  physics. 

This  new  science  always  had  its  cultivators,  even  before 
it  had  a  name.  The  elder  Herschel  set  himself  the  problem 
"  to  find  out  the  construction  of  the  heavens/'  and  this  is 
the  problem  of  astrophysics,  in  contradistinction  to  the 
problem  of  exact  astronomy — "  to  find  out  how  the  heav- 
enly bodies  move."  The  modern  form  of  Herschel's  phrase 
is,  "  to  determine  the  present  constitution  and  the  evolu- 
tion history  of  the  stars,  the  comets,  the  sun,  the  planets." 

We  must  regard  Sir  William  Herschel  as  the  founder  of 
the  science.  He  has  had  great  followers:  Schroeter,  Sir 
John  Herschel,  Beer,  Maedler,  Fraunhofer,  Kirchhoff, 
Bunsen,  Lassell,  Bond,  De  la  Rue,  Rutherfurd,  Draper, 
Schiaparelli,  Vogel,  Janssen,  Lockyer,  Young,  Langley, 
Pickering,  not  to  speak  of  a  host  of  other  familiar  names. 

To-day  there  are  several  observatories  devoted  exclu- 
sively to  the  new  science  and  their  number  is  growing. 
This  should  be  so.  There  are  too  many  astronomical  ob- 
servatories idle.  If  the  charm  of  the  new  fields  is  enough 
to  make  them  efficient  in  forwarding  the  science  as  a  whole, 
we  must  welcome  the  new  impetus.  But  there  is  a  note 
of  warning  to  which  we  must  give  attention.  We  must 
keep  strictly  before  us  the  means  by  which  the  older  astron- 
omy has  arrived  at  its  proud  position  as  the  chief  of  the 
physical  sciences.  For  hundreds,  yes,  thousands  of  years, 
one  principle  has  run  through  all  of  astronomy.  Assidu- 
ous observations  must  be  made  according  to  well-consid- 
ered plans,  matured  after  deep  reflection.  The  results  of 
these  observations  must  be  compared  with  a  theory  ex- 
pressed rigorously  in  the  terms  of  mathematics.  The  dif- 
ferences between  observation  and  theory  must  be  treated 
by  a  profound  analysis,  so  to  derive  corrections  to  the  pro- 
visional theory.  This  provisional  theory  will  in  its  turn 
become  the  basis  of  comparison  with  Nature,  and  so  on, 


292  HOLDEN 

until  the  ideal  is  reached  by  successive  approximations. 
This  ideal  is  simple,  and  in  many  researches  it  has  been 
attained  already.  It  is  to  push  the  successive  approxima- 
tions until  we  can  predict  the  position  or  the  motion  of  a 
heavenly  body  as  accurately  as  we  can  observe  it.  When 
this  stage  is  reached  we  may  leave  the  special  problem  in 
hand,  until  the  methods  of  observation  are  themselves  im- 
proved. 

If  astrophysics  will  accept  this  ideal  and  strive  for  it, 
there  is  no  future  so  brilliant  that  we  may  not  claim  it  for 
her  portion.  If  this  straight  and  narrow  way  is  departed 
from,  although  the  new  science  is  followed  never  so  assidu- 
ously, no  essential  progress  can  be  expected,  and  real  harm 
is  sure  to  follow. 

Astrophysics  has  three  well-marked  lines  of  research — 
namely:  spectrum  analysis  (now  a  quarter  of  a  century  old), 
celestial  photometry  (half  a  century),  celestial  photography 
(dating  back  exactly  forty-six  years).  Schiaparelli's  theory 
of  meteor-streams  and  their  connection  with  comets  be- 
longs to  this  science  in  so  far  as  it  throws  light  upon  the 
material  out  of  which  comets  are  built;  and  every  part  of 
physics  which  treats  of  the  action  of  one  body  upon  an- 
other body  at  a  distance,  whether  through  gravitation, 
heat,  magnetism,  electricity,  has  close  relations  to  it.  But 
the  three  main  paths  are  spectroscopy,  celestial  photometry, 
and  celestial  photography.  It  is  of  the  latter  path  that  I 
wish  to  speak  in  this  paper.  We  shall  follow  it  assiduously 
at  the  Lick  Observatory,  and  we  shall  have  unrivalled  op- 
portunities to  do  so. 

Spectroscopy  in  certain  of  its  lines  we  shall  also  follow, 
and  our  opportunities  in  this  branch  also  are  unique. 
Photometry  is  so  thoroughly  done  at  the  Harvard  College 
Observatory  that  it  would  be  a  waste  of  energy  for  another 
American  observatory  to  devote  any  great  part  of  its  time 
to  such  researches. 

I  assume  that  some  slight  explanation  of  the  differences 
between  a  photographic  telescope  and  an  ordinary  one  will 
not  be  superfluous.  The  object-glass  of  an  ordinary  tele- 


PHOTOGRAPHY  THE  SERVANT  OF  ASTRONOMY  293 

scope  brings  the  rays  by  which  we  see  (those  having  a  wave- 
length of  about  6,000  ten-millionths  of  a  millimetre)  to 
an  accurate  focus.  These  can  not  be  photographed  except 
by  special  plates  and  with  special  difficulty.  The  rays  which 
affect  the  photographic  salts  of  silver  have  a  wave-length 
of  about  4,000  ten-millionths  of  a  millimetre,  and  to  bring 
these  to  a  focus  the  two  lenses  of  the  ordinary  achromatic 
object-glass  must  be  supplemented  by  a  third  lens.  This 
third  lens  is  so  arranged  that  it  can  be  placed  in  front  of 
(and  close  against)  the  ordinary  objective,  and  it  turns  the 
telescope  from  a  seeing  instrument  into  a  camera.  It  is 
also  necessary  to  say  that  if  the  telescope  remains  fixed, 
while  a  bright  star  is  passing  across  its  field  of  view,  the 
image  of  the  star  will  pass  across  the  sensitive  plate,  and 
will  leave  a  "  trail "  which  is  the  visible  representative  of 
the  direction  of  the  star's  diurnal  motion.  Equatorial  stars 
as  faint  as  the  eighth  or  ninth  magnitude  will  give  trails. 

If,  on  the  contrary,  we  attach  an  accurate  driving  clock 
to  the  telescope,  and  cause  it  to  follow  the  star  in  its 
motion  from  east  to  west,  rising  to  setting,  we  shall  have 
instead  of  a  trail  a  bright  point,  the  photographic  image. 
If  we  wish  to  make  a  picture  of  the  sky,  we  must  register 
the  stars  by  such  points  as  these.  The  trails  have,  however, 
various  advantages,  one  of  which  is  that  they  can  not  be 
mistaken  for  dust  or  for  pin  holes  on  the  plate  itself.  The 
position  of  the  dots  in  latitude  and  longitude  can  be  very 
accurately  measured.  The  latitude  of  the  star  can  be  even 
better  determined  from  its  trail,  but  its  longitude  must  then 
be  determined  by  special  devices,  which  I  need  not  de- 
scribe. In  the  ordinary  methods  of  observing,  the  astron- 
omer views  the  visual  images  of  the  heavenly  bodies,  and 
either  examines  their  surfaces,  or  determines  their  position 
with  reference  to  adjacent  bodies  (as,  for  example,  the  posi- 
tions of  satellites  relative  to  their  planet),  by  means  of  ex- 
tremely accurate  and  refined  micrometers,  forming  a  part 
of  the  eyepiece  of  his  telescope. 

To  utilize  photographic  plates  fully,  and  especially  to 
make  them  a  substitute  for  micrometric  measures,  it  is 


294 


HOLDEN 


necessary  to  contrive  elaborate  measuring  engines  to  take 
the  place  of  the  costly  micrometers,  ordinarily  used  with 
telescopes.  These  engines  measure  the  positions  of  the 
dots  or  trails  on  the  plates,  after  these  have  been  removed 
from  the  telescope. 

Mr.  Rutherfurd  first  made  a  satisfactory  engine  of  this 
kind;  it  was  then  improved  upon  in  the  design  of  Pro- 
fessor Harkness  adopted  by  the  United  States  Transit  of 
Venus  Commission,  in  1874,  and  the  Lick  Observatory 
owns  the  finest  specimen  of  this  class,  which  was  made  for 
it  under  the  supervision  of  Professor  Harkness. 

The  very  first  essay  in  astronomical  photography  was 
that  of  Professor  John  William  Draper,  of  New  York,  who, 
in  the  year  1840,  took  a  satisfactory  daguerreotype  of  the 
moon.  The  experiments  of  Dr.  Draper  were  repeated  by 
George  Bond,  Director  of  the  Harvard  College  Observa- 
tory, in  1850,  and  a  lunar  daguerreotype  made  by  him  was 
exhibited  at  London  in  1851,  at  the  World's  Fair,  where 
it  attracted  much  attention. 

During  the  years  1853  to  1857,  Mr.  De  la  Rue,  of 
London,  made  lunar  daguerreotypes  and  photographs, 
some  of  great  excellence.  In  1864  Dr.  Lewis  Rutherfurd, 
of  New  York,  made  an  eleven-and-a-half-inch  objective, 
which  was  corrected  only  for  the  photographic  rays,  and  by 
means  of  this  he  obtained  the  finest  photographs  of  the 
moon  which  had  yet  been  made.  Dr.  Henry  Draper, 
about  the  same  time,  made  a  fifteen-inch  reflecting  tele- 
scope with  which  he  also  took  excellent  lunar  photographs. 
These  latter  have  been  enlarged  to  three  and  even  to  four 
feet  in  diameter,  from  the  original  picture  of  about  two 
inches  and  a  half.  A  long-focus  telescope  is  of  great  ad- 
vantage in  these  researches.  The  pictures  in  the  principal 
focus  of  the  Melbourne  reflector  are  some  six  inches  in 
diameter,  and  I  have  seen  a  few  of  these  of  great  excellence. 
Such  pictures  can  be  enlarged,  in  printing,  from  six  to 
twelve  times. 

The  photographs  of  the  moon  in  the  focus  of  the  Lick 
equatorial  will  be  six  inches  in  diameter,  and  will  probably 


PHOTOGRAPHY  THE   SERVANT   OF   ASTRONOMY 


295 


stand  an  enlargement  of  twelve  times,  so  as  to  be  six  feet 
finally. 

Lunar  photographs  have  not  advanced  our  knowledge 
in  any  important  degree  up  to  this  time,  however. 

Solar  daguerreotypes  were  first  taken  by  Foucault  and 
Fizeau,  in  1845,  at  Paris,  on  the  advice  of  Arago.  In  1857 
Mr.  De  la  Rue  contrived  the  photoheliograph  for  the  Kew 
Observatory,  by  which  solar  photographs  have  been  taken 
since  that  time  daily  at  Kew  and  Greenwich. 

M.  Janssen,  of  Mendon,  near  Paris,  about  1878,  succeed- 
ed in  making  his  exquisite  photographs  of  the  sun  on  glass, 
which  show  an  astonishing  amount  of  detail.  I  understand 
that  these  are  chiefly  made  by  means  of  a  six-inch  refractor, 
and  I  have  never  been  able  to  comprehend  how  so  much  de- 
tail can  be  shown  writh  an  objective  of  such  a  small  separat- 
ing power,  nor  to  rid  myself  of  an  impression  that  some,  at 
least,  of  these  details  are  due  to  atmospheric  disturbances. 

If  the  exposures  are  made  extremely  short  (^-jrVir  to 
Tinnr  °f  a  second),  very  successful  results  can  be  obtained 
in  solar  photography.  There  is,  undoubtedly,  an  important 
field  of  research  still  open  here,  especially  with  large  ob- 
jectives of  great  separating  power. 

The  first  photographs  of  a  solar  eclipse  were  made  by 
Busch,  at  Konigsberg,  in  1851,  and  by  Bartlett  at  West 
Point,  in  1854.  The  eclipse  photographs  of  Secchi  and 
De  la  Rue,  in  1860,  were  of  high  scientific  importance, 
since  they  established  beyond  doubt  the  fact  that  the  solar 
protuberances  were  really  appendages  of  the  sun,  and  not 
of  the  moon. 

I  believe  the  first  photograph  of  the  spectrum  of  the 
sun  at  a  solar  eclipse  was  taken  at  the  Egyptian  eclipse  of 
1882,  by  Professor  Schuster,  and  also  by  the  party  under 
Mr.  Lockyer.  Very  perfect  photographs  of  the  solar  spec- 
trum were  taken  at  the  total  eclipse  of  1883  in  the  Pacific 
Ocean,  by  the  English  parties  and  by  the  French  parties, 
and  the  subject  does  not  now  present  any  great  difficulties. 

Photography  served  a  very  useful  purpose  in  its  appli- 
cation to  the  transits  of  Venus  of  1874  and  1882. 


296 


HOLDEN 


According  to  Professor  Pickering1,  the  first  daguerreo- 
type of  a  star  was  taken  at  Harvard  College  Observatory, 
on  July  17,  1850,  under  the  direction  of  the  elder  Bond. 
The  star  Vega  was  satisfactorily  daguerreotyped,  and  later 
the  double  star  Castor  gave  an  elongated  image,  which  was 
plainly  due  to  its  two  components.  The  sensitiveness  of 
the  daguerreotype  plates  then  in  use  was  so  small  that  even 
such  bright  stars  as  these  gave  faint  images,  and  no  im- 
pression whatever  was  obtained  from  the  Pole  Star,  no  mat- 
ter how  long  the  exposure.  These  experiments  were  re- 
peated with  various  stars  and  clusters,  but  finally  the  work 
was  abandoned  on  account  of  photographic  difficulties.  In 
1857  the  younger  Bond  resumed  the  research.  At  this 
time  the  collodion  process  had  greatly  reduced  the  time  of 
exposure,  and  the  plates  were  of  much  greater  sensitive- 
ness. An  impression  of  the  double  star  Zeta  Ursse  Majoris 
was  obtained  in  eight  seconds.  A  trail  was  obtained  from 
the  image  of  the  bright  star  Vega.  The  faintest  star  photo- 
graphed was  the  companion  of  Epsilon  Lyrae,  which  is  of 
the  sixth  magnitude — that  is,  just  visible  to  the  naked  eye. 

A  series  of  measures  was  made  of  the  relative  positions 
and  distances  of  the  various  double  stars  photographed,  in 
order  to  see  whether  measures  made  upon  a  photographic 
plate  could  be  used  to  replace  those  made  in  the  ordinary 
manner  at  the  telescope.  It  was  found  that  a  single  measure 
made  upon  the  plate  was  about  of  the  same  value  as  a 
single  measure  made  by  an  astronomer  with  the  ordinary 
micrometer.  Professor  Bond  pointed  out  very  clearly  how 
photographic  images  might  be  used  to  determine  accu- 
rately the  relative  brightness  of  stars,  and  also  what  the  ad- 
vantages of  photography  were  for  the  permanent  registra- 
tion of  star  positions.  Mr.  De  la  Rue  and  Dr.  Rutherfurd 
soon  after  repeated  these  experiments  of  Professor  Bond, 
and  a  very  extended  investigation  was  undertaken  in  1864 
by  Dr.  Rutherfurd,  and  continued  by  him  for  many  years. 
Most  of  the  principal  clusters  in  the  northern  heavens  were 
photographed,  as  well  as  most  of  the  brighter  double  stars. 
These  researches  have  never  been  fully  utilized  for  the 


PHOTOGRAPHY   THE   SERVANT   OF   ASTRONOMY   297 

following  reason:  the  photographs  were  measured  in  the 
most  careful  manner  on  a  measuring  engine,  in  which  the 
distances  of  one  star  from  another  were  determined  by 
means  of  a  very  accurate  screw.  After  the  series  of  meas- 
ures had  been  continued  for  several  years,  it  was  discovered 
that  the  screw  itself  had  worn  considerably,  so  that  the 
value  of  its  revolutions  was  not  the  same  as  it  had  for- 
merly been.  It  was  impossible  to  discover  at  what  time 
this  wear  commenced,  or  how  it  progressed,  and  therefore 
these  excellent  photographs  have  remained  undiscussed  up 
to  the  present  time.2  The  distances,  which  must  be  accu- 
rately measured,  are  about  so}00  of  an  inch.  The  faintest 
stars  shown  in  Dr.  Rutherfurd's  eleven-inch  telescope  are 
about  of  the  ninth  magnitude.  The  plates  used  by  Dr. 
Rutherfurd  were,  I  believe,  exclusively  wet  plates. 

Dr.  Henry  Draper  attacked  the  same  problem  in  1880, 
using,  however,  the  most  sensitive  dry  plates  then  available. 
In  1 88 1,  with  an  eleven-inch  refractor  constructed  by  the 
Clarks,  he  obtained  a  photograph  of  the  nebula  in  Orion, 
in  which  one  of  the  stars  is  shown  whose  magnitude  is 
not  more  than  14^.  This  star  is  barely  visible  with  a  tele- 
scope of  the  same  aperture  as  that  with  which  the  photo- 
graph was  taken.  The  photographic  plate  now  had  be- 
come as  efficient  an  instrument  of  research  as  the  eye  itself. 
M.  Janssen  also  photographed  the  nebula  in  Orion  in 
1 88 1,  but  the  best  of  all  such  photographs  has  been  made 
by  Mr.  Common,  of  England,  with  his  three-foot  silver-on- 
glass  reflector. 

Dr.  B.  A.  Gould,  in  his  expedition  to  the  southern 
hemisphere  (i87O-'84),  carried  with  him  a  photographic 
lens  of  eleven  inches  aperture,  and  during  his  entire  stay 
of  more  than  ten  years  employed  all  the  available  time 
at  his  command  in  accumulating  negatives  of  the  principal 
southern  double  stars  and  clusters.  These  photographs 
have  not  yet  been  discussed,  and  Dr.  Gould  has  discovered 
that  there  are  signs  that  the  films  on  the  negatives  are  now 

1  They  are  now  (1900)  being  remeasured  and  rediscussed  at  Colum- 
bia University. 


298  HOLDEN 

beginning  to  deteriorate.  Probably  this  extensive  and  im- 
portant series  will  soon  receive  discussion.3 

During  the  years  i882-'86  many  observatories  have 
undertaken  some  researches  in  stellar  photography.  The 
Royal  Astronomer  at  the  Cape  of  Good  Hope,  Dr.  Gill, 
has  undertaken  to  make  a  map  of  the  whole  southern 
heavens  by  photographic  means  only.  The  Rev.  T.  E. 
Espin,  of  Liverpool,  has  published  a  catalogue  of  the  mag- 
nitudes of  500  stars,  determined  by  means  of  photography 
alone.  The  most  extensive  investigation  is  that  of  the 
brothers  Paul  and  Prosper  Henry,  of  the  Observatory  of 
Paris.  Important  investigations  have  also  been  made  at 
the  Astrophysical  Observatory  of  Potsdam  and  at  two 
Physical  observatories  in  Hungary. 

In  1863  Dr.  Huggins,  of  London,  obtained  a  photo- 
graphic image  of  the  spectrum  of  Sirius,  but  no  lines  were 
visible  in  this  spectrum.  The  first  successful  photograph  of 
the  spectrum  of  a  star  was  obtained  by  Dr.  Henry  Draper  in 
1872.  Both  of  these  astronomers  succeeded  in  1876  in 
obtaining  valuable  spectrum  photographs  of  the  brightest 
stars.  In  1882  they  both  obtained  a  photograph  of  the 
spectrum  of  the  nebula  in  Orion.  Since  1882  many  astron- 
omers and  observatories  have  devoted  themselves  to  pho- 
tographic researches,  but  little  has  been  published,  except 
by  the  Observatory  of  Harvard  College.  Here  the  years 
1882-^85  were  spent  in  very  elaborate  experiments,  prelimi- 
nary to  undertaking  larger  and  more  important  researches. 
A  very  large  number  of  photographs  have  been  taken  of 
the  regions  lying  about  the  north  celestial  pole.  The  pho- 
tographic telescope  employed  is  eight  inches  in  aperture. 
The  chief  results  up  to  now  have  been  the  establishing  the 
relative  brightness  of  117  stars  within  one  degree  of  the 
pole.  A  second  research  in  progress  is  the  determination 
of  the  relative  brilliancy  of  all  the  brighter  stars.  Other 
experiments  are  in  hand,  but,  as  they  all  relate  to  pho- 
tometry or  to  spectroscopy,  they  may  be  passed  over  here, 

8  This  work  has  lately  been  published  under  the  direction  of  Dr. 
S.  C.  Chandler. 


PHOTOGRAPHY  THE   SERVANT  OF   ASTRONOMY  299 

after  merely  calling  attention  to  them  as  the  most  impor- 
tant researches  of  the  kind. 

In  1882  Dr.  Gill,  at  the  Cape  of  Good  Hope,  succeeded 
in  photographing  the  great  comet  of  that  year,  and  in 
doing  this  he  proved  the  practicable  possibility  of  making 
star  rnaps,  which  should  contain  all  the  stars  down  to  the 
tenth  magnitude.  In  1885  the  Royal  Society  granted  £300 
to  the  Cape  of  Good  Hope  Observatory  for  photographic 
purposes.  Dr.  Gill  has  set  himself  to  the  solution  of  two 
problems:  First,  that  of  securing  as  soon  as  possible  a 
complete  photographic  map  of  the  southern  heavens,  con- 
taining every  star  visible  down  to  the  tenth  magnitude, 
so  as  to  continue  the  "  Durchmusterung  "  of  Argelander; 
secondly,  to  test  the  possibility  of  photographing  the  solar 
corona  daily  without  an  eclipse,  by  the  method  first  sug- 
gested by  Dr.  Huggins.  For  the  first  purpose  Dr.  Gill 
makes  use  of  one  of  Dallmeyer's  rapid  rectilinear  combina- 
tions, composed  of  two  concavo-convex  achromatic  com- 
binations of  six  inches  aperture.  This  camera  is  mounted 
on  an  equatorial  stand,  and  is  pointed  by  means  of  a  tele- 
scope of  forty-five  inches  focal  length  and  three  and  a 
half  inches  aperture.  The  exposures  are  an  hour  long 
when  the  sky  is  clear.  Each  plate  is  six  inches  square,  and 
covers  an  area  of  about  thirty-six  degrees.  Every  such 
area  is  photographed  twice,  so  as  to  render  it  impossible  to 
confound  the  images  of  faint  stars  with  minute  dust  specks. 
In  this  way  a  great  portion  of  the  sky  has  already  been 
photographed  in  duplicate. 

The  same  observatory  has  recently  obtained  a  much 
more  powerful  optical  apparatus  through  the  generosity  of 
Mr.  James  Nasmyth,  who  has  purchased  a  specially  cor- 
rected photographic  objective  of  nine  inches  aperture  and 
nine  feet  focal  length,  made  by  Mr.  Grubb,  of  Dublin.  The 
field  of  this  Nasmyth  lens  will  be  much  more  limited  than 
that  of  the  Dallmeyer  apparatus,  but  it  is  expected  to  ob- 
tain from  it  a  photograph  of  all  stars  to  the  twelfth  or  thir- 
teenth magnitude  inclusive  within  a  circle  of  a  radius  of  one 
or  one  and  a  half  degree. 


HOLDEN 

Mr.  Roberts,  in  England,  has  erected  a  reflector  of 
twenty  inches  aperture  and  of  one  hundred  inches  focus 
for  stellar  photography  alone,  and  has  made  considerable 
progress  in  the  work  of  charting  the  northern  heavens. 
The  size  of  the  field  of  Mr.  Roberts's  telescope  is  two  de- 
grees in  declination  and  one  and  a  half  degree  in  right 
ascension.  The  time  of  exposure  is  fifteen  minutes  in  a 
clear  sky.  The  companion  to  the  Pole  Star  is  just  visible 
in  four  seconds  under  the  best  circumstances.  Mr.  Roberts 
refers  to  an  important  difficulty,  which  is,  that  in  most 
photographic  plates  there  are  small  specks  in  the  film, 
many  of  which  look  like  stars,  and  which  are  extremely 
difficult  to  distinguish  from  stars  even  when  they  are 
viewed  through  a  microscope.  Dr.  Gill,  at  the  Cape  of 
Good  Hope,  avoids  this  difficulty  by  taking  two  photo- 
graphs of  the  same  field  successively,  giving  to  each  an 
exposure  of  one  hour.  At  Paris  three  exposures  of  an 
hour  each  are  made,  on  the  same  plate. 

Mr.  Common's  experiments  commenced  in  1879.  At 
this  time,  using  dry  plates  with  his  three-foot  reflector,  he 
took  successful  pictures  of  the  Pleiades,  with  one  and  a 
half  minute  exposure,  showing  all  the  stars  to  the  eighth 
and  ninth  magnitude.  In  1882  he  devoted  his  time  to  pho- 
tographing the  nebula  in  Orion,  and  has  obtained  won- 
derful results. 

After  making  such  a  splendid  success  with  his  three- 
foot  reflector,  Mr.  Common  is  now  making  one  of  five  feet 
in  aperture.  There  is  no  doubt  that  a  mirror  of  this  aper- 
ture can  be  accurately  figured  by  the  optician.  The  dif- 
ficulties in  using  it  come  from  unequal  flexure  of  its  various 
parts  and  from  their  differing  temperatures.  Difficulties 
of  this  nature  have  never  yet  been  successfully  overcome 
for  reflectors  of  more  than  thirty-six  inches  of  aperture,  but 
Mr.  Common's  great  mechanical  skill  and  knowledge  and 
experience  lead  us  to  hope  that  he  may  succeed  in  this 
important  undertaking. 

In  September,  1884,  Dr.  Lohse  used  the  eleven-inch 
refractor  of  the  Potsdam  Observatory  to  photograph  the 


PHOTOGRAPHY  THE   SERVANT  OF   ASTRONOMY  301 

star  cluster  in  Perseus.  An  exposure  of  forty-five  minutes 
was  given,  and  stars  as  faint  as  the  tenth  and  eleventh  mag- 
nitude were  registered. 

A  number  of  other  star  clusters  have  also  been  photo- 
graphed by  Dr.  Lohse.  The  Savilian  Observatory  at  Ox- 
ford (England)  has  undertaken  to  study  two  constellations 
(Lyra  and  Cassiopeia)  by  photography  on  plates  one  de- 
gree square. 

The  early  experiments  at  the  Paris  Observatory,  1884, 
were  made  with  a  telescope  with  an  aperture  of  -^^  of  a 
metre  (6.3  inches),  and  they  were  so  successful  that  it  was 
decided  to  make  a  larger  instrument  specially  for  photog- 
raphy, and  soon  an  objective  of  -fl^  of  a  metre  aperture 
(13.4  inches)  and  3 -^metres  focal  length  (134  inches)  was 
made.  Parallel  to  this  photographic  telescope,  one  of  about 
the  same  focus  and  of  -ffo  of  a  metre  (9.5  inches)  aperture 
is  placed  as  a  directing  telescope.  In  May,  1885,  the  new 
photographic  telescope  was  first  brought  into  use,  and  a 
few  of  the  important  results  that  have  been  reached  by  it 
are  mentioned  below.  Stars  down  to  the  fifteenth  mag- 
nitude are  photographed  with  an  exposure  of  one  hour, 
the  plates  being  something  more  than  two  degrees  square. 
From  one  to  two  thousand  stars  are  shown  to  each  square 
degree  with  this  exposure,  using  dry  plates.  On  these 
plates  three  separate  exposures  of  an  hour  each  are  given, 
the  instrument  being  moved  between  each  exposure,  so 
as  to  change  the  position  of  the  image  on  the  plate  about 
five  seconds  of  arc  each  time.  The  three  images  of  the 
same  star  thus  form  a  little  triangle.  By  means  of  this  tele- 
scope a  new  and  very  faint  nebula  has  been  discovered  in 
the  Pleiades,  which  would  never  have  been  discovered  if 
we  depended  on  the  eye  alone.  Admirable  photographs 
of  Saturn  have  been  taken  by  direct  enlargement  of  the 
primary  image,  through  a  non-achromatic  eyepiece,  which 
gives  a  magnifying  power  of  eleven  times.  Hyperion,  the 
faintest  satellite  of  Saturn,  a  difficult  object  in  the  twenty- 
six-inch  telescope  at  Washington,  has  been  photographed 
with  an  exposure  of  thirty  minutes,  and  the  satellite  of 


302 


HOLDEN 


Neptune  can  be  taken  in  any  part  of  its  orbit.  With  an 
exposure  of  one  hour  the  eleventh-  and  fifteenth-magni- 
tude stars  have  an  actual  diameter  of  about  16*00  of  an 
inch — that  is,  in  arc  about  one  and  a  half  second.  Stars 
of  the  fifth  or  sixth  magnitude  are  about  one  minute  in 
diameter  with  long  exposures.  With  a  properly  limited 
exposure,  these  also  are  of  extremely  minute  dimen- 
sions. 

The  proper  exposure  for  a  first-magnitude  star,  like 
Sirius  or  Vega,  is  not  more  than  y^  of  a  second.  For 
a  star  just  visible  to  the  naked  eye,  half  a  second  is  suf- 
ficient. For  stars  of  the  tenth  magnitude,  twenty  sec- 
onds; of  the  twelfth,  two  minutes;  of  the  thirteenth,  five 
minutes;  of  the  fourteenth,  thirteen  minutes;  and  for  the 
faintest  visible,  an  hour  and  twenty-three  minutes.  These 
results  are,  of  course,  a  minimum,  and  but  approximate. 

So  far  as  is  known,  the  growth  of  the  image  of  a  star 
upon  the  photographic  plate  is  equal,  and  concentric  with 
the  point  of  the  plate,  on  which  the  image  of  the  star  falls. 
The  faintest  stars  on  these  Paris  plates  are,  as  was  said, 
arranged  in  little  groups  of  three.  As  the  brighter  stars  on 
these  plates  are  examined,  it  is  found  that  the  size  of  each 
of  the  images  increases  until  they  overlap;  this  continues 
until  the  complete  images  of  any  one  bright  star  are  very 
much  larger  than  the  original  small  triangle.  The  appear- 
ance is  as  if  three  circles  nearly  as  large  as  the  resulting 
image  had  been  struck  from  the  centre  of  what  would  have 
been  (in  the  case  of  a  smaller  star)  the  three  separate  stars 
of  a  group.  It  is  the  intention  of  the  director  of  the  Ob- 
servatory of  Paris  to  use  this  photographic  telescope  to 
continue  the  construction  of  ecliptic  charts.  And  it  is 
suggested  by  the  director  that  by  means  of  the  co-opera- 
tion of  six  or  eight  observatories  the  whole  heavens  should 
be  charted  in  a  similar  way.  There  are  41,000  square  de- 
grees in  the  whole  heavens,  and  if  six  square  degrees  can 
be  registered  on  a  plate  (with  one  hour's  exposure),  7,000 
such  plates  must  be  made,  requiring  at  least  7,000  hours. 
To  avoid  mistakes,  at  least  two  exposures  must  be  given 


PHOTOGRAPHY   THE   SERVANT   OF  ASTRONOMY   303 

for  each  region,  or  14,000  plates  and   14,000  hours  are 
necessary. 

,i.  $  If  we  allow  loo  clear  nights  in  a  year  (which  is  a  fair 
allowance  for  all  observatories  except  the  Lick  Observa- 
tory, where  we  can  count  on  at  least  200),  it  would  require 
140  years  at  any  one  observatory  to  do  this  work,  or  14 
at  ten  observatories.  I  personally  doubt  whether  the  strict 
adherence  to  a  plan,  which  is  indispensable  to  success, 
could  be  maintained  at  so  many  establishments  for  so  long 
a  period.  The  whole  subject  is  yet  in  too  unsettled  a  state 
to  warrant  an  international  undertaking  of  such  magnitude 
at  present.  A  number  of  years  must  be  spent  in  tentative 
researches  before  the  right  paths  are  struck  out.  I  give 
some  of  the  most  obvious  directions  for  these  trials  in  what 
follows. 

The  two  hundred  and  sixty  or  more  small  planets 
(asteroids)  which  lie  between  Mars  and  Jupiter  have  all 
been  discovered  by  the  slow  process  of  comparing  a  star 
map,  night  after  night,  with  the  heavens.  A  star  not  on 
the  map  is  either  an  omitted  star  to  be  inserted,  or  a  minor 
planet,  known  or  unknown.  A  photographic  objective  of 
twelve  inches  aperture  will  show  a  trail  for  a  star  of  the 
magnitude  of  the  brighter  asteroids  with  an  exposure  of 
half  an  hour.  An  hour's  exposure  will  probably  show  the 
trail  of  the  faintest  asteroids  (twelve  to  thirteen  magnitude). 
One  of  the  immediate  results  of  the  application  of  photog- 
raphy will  undoubtedly  be  to  greatly  increase  the  number 
of  known  asteroids. 

There  are  reasons  to  believe  in  the  existence  of  a  major 
planet  exterior  to  Neptune.  If  such  a  planet  exists  it  is 
not  likely  to  be  brighter  than  the  tenth  magnitude,  and  its 
motion  will  be  very  slow.  Hence  it  is  unlikely,  at  least, 
that  such  a  planet  can  be  discovered  by  its  trail  on  the  plate. 
The  method  of  three  exposures  on  the  same  plate  employed 
at  Paris  probably  would  not  disclose  the  existence  of  a 
trans-Neptunian  planet,  though  it  would  suffice  for  the  de- 
tection of  Neptune  itself  in  most  parts  of  its  orbit.  Prob- 
ably the  surest  way  to  detect  such  a  body,  if  it  exists, 


304 


HOLDEN 


would  be  to  take  photographs  of  the  same  region  on  suc- 
cessive days.  Such  plates  would  then  have  to  be  laboriously 
compared,  star  by  star.  Doubtful  cases  would  require  a 
third  night's  work  to  be  done  in  order  to  decide. 

A  blue-print  of  two  such  plates  will  enable  all  the 
brighter  stars  to  be  quickly  compared  and  disposed  of. 
The  real  labour  will  then  be  confined  to  the  stars  less 
bright  than  the  faintest  which  can  be  blue-printed.  The 
problem  of  the  constitution  of  the  stellar  universe  must  be 
studied,  it  seems,  by  some  kind  of  celestial  statistics  de- 
rived from  counts  or  gauges  of  the  stars.  Nearly  all  the 
conclusions  we  have  so  far  reached  are  based  on  the  counts 
made  by  Sir  William  Herschel.  I  have  myself  spent  much 
time  in  continuing  these.  All  such  work  is  now  useless. 
Photographic  maps  will  give  us  all  the  requisite  data,  and 
will  throw  much  light,  too,  on  another  closely  connected 
problem — the  extinction  of  light  in  space — provided  only 
that  all  negatives  taken  for  this  object  are  made  strictly 
comparable  in  every  respect.  This  proviso  is  of  the  utmost 
importance,  and  very  difficult  to  be  lived  up  to  in  any 
work  done  by  co-operating  observatories.  It  is  just  pos- 
sible that  photometric  measures  of  the  photographs  of  a 
very  eccentric  asteroid  can  now  be  made  with  sufficient 
delicacy  to  settle  the  question  whether  light  is,  or  is  not, 
extinguished  in  space. 

The  precision  of  the  photographic  images  of  stars  is  so 
great  that  there  is  no  doubt  that  measures  of  the  negatives 
of  double  stars,  of  star  clusters  and  groups,  will,  at  least  in 
most  instances,  take  the  place  of  the  painful  and  laborious 
micrometric  measures  which  are  now  employed  by  ob- 
servers. The  photographs  have  their  own  errors,  and 
many  of  them;  but  these  are  all  susceptible  of  investi- 
gation. 

The  shrinkage  of  the  gelatine  films  of  the  negatives  is 
likely  to  prove  a  grave  difficulty  in  the  application  of  pho- 
tography to  exact  astronomy,  but  this  can  always  be  de- 
tected by  photographing  a  network  of  lines  on  glass.  Very 
serious  difficulties  of  this  kind  have  lately  been  met  with 


PHOTOGRAPHY   THE   SERVANT   OF  ASTRONOMY   305 

by  Professor  Pritchard,  of  Oxford,  in  his  researches  on  the 
(photographic)  parallax  of  61  Cygni. 

But  photographic  plates  have  also  many  capital  advan- 
tages. For  example,  the  photographic  impress  of  a  star 
gives  really  its  mean  or  average  position,  freed  from  those 
accidental  and  transitory  variations  of  place  which  are  due 
to  variations  of  atmospheric  refraction — a  constant  source 
of  error.  The  saving  of  time  is  also  important. 

An  exposure  of  an  hour  has  given  (at  the  Paris  Observa- 
tory) a  map  of  5,000  stars  in  four  square  degrees  in  the 
constellation  Cygnus.  The  best  maps  we  now  have  give 
170  of  the  brightest  stars  only  in  this  place.  To  map  5,000 
stars  by  the  eye  alone  would  require  several  years.  The 
writer  spent  all  the  time  he  could  spare  from  routine  ob- 
servations during  four  years  with  the  twenty-six-inch  equa- 
torial, at  Washington,  in  a  study  of  the  nebula  in  Orion. 
Every  important  result  reached  by  that  study,  and  very 
many  not  comprised  in  it,  was  attained  by  Mr.  Common's 
photograph  (subsequently  taken),  which  required  an  ex- 
posure of  forty  minutes  only. 

Another  important  advantage  of  the  new  methods  is 
that  they  do  not  require  highly  skilled  observers.  It  re- 
quired a  Bessel  or  a  Struve  to  determine  the  parallax  of  61 
Cygni  or  of  Vega.  But  photographic  exposures  can  be 
made,  and  glass  negatives  successfully  measured  by  well- 
trained  assistants,  after  the  plan  of  observation  has  once 
been  thoroughly  thought  out.  This  is  no  slight  benefit. 
The  skill  of  the  astronomer  is  reserved  for  real  difficulties, 
and  the  merely  laborious  work  can  be  done  in  duplicate, 
if  necessary,  by  younger  men. 

Again,  the  chemical  plate  is  sensitive  to  a  whole  series 
of  rays,  which  produce  no  effect  on  the  human  eye.  Only 
half  of  the  faintest  stars  of  any  photographic  map  are  visible 
to  the  eye  in  the  same  telescope.  Photographic  methods 
thus  increase  the  range  of  our  vision  immensely;  they  also 
increase  its  sharpness.  The  photographic  plate  will  regis- 
ter the  sum  of  all  the  impressions  it  receives.  It  does  not 
tire,  as  the  eye  does,  and  refuse  to  pay  attention  for  more 

20 


306  HOLDEN 

than  a  small  fraction  of  a  second,  but  it  will  faithfully 
record  every  ray  of  light  that  falls  upon  it,  even  for  hours, 
and  finally  it  will  produce  its  automatic  register,  so  that 
the  eye  can  see  it,  and  so  that  this  can  be  measured,  if 
necessary,  again  and  again.  The  permanence  of  the  records 
is  of  the  greatest  importance,  and,  so  far  as  we  know,  it 
is  complete  when  the  best  modern  plates  are  employed. 

We  can  hand  down  to  our  successors  a  picture  of  the 
sky  locked  in  a  box.  What  would  we  not  give  for  such 
a  record  bequeathed  to  us  by  Hipparchus  or  by  Galileo! 

It  will  be  of  interest  to  briefly  state  here  how  far  the 
equipment  of  the  Lick  Observatory  will  fit  it  to  engage  in 
this  important  branch  of  research.  It  is  known  that  the 
situation  of  the  observatory  is  the  finest  in  the  world,  both 
as  to  the  number  of  clear  days  and  as  to  the  quality  of  steady 
atmosphere.  The  observatory  will  be  completely  equipped 
for  all  micrometric  work,  and  also  for  all  spectroscopic 
researches. 

Mr.  Lick's  bequest  for  the  observatory  was  $700,000, 
of  which  nearly  $200,000  will  remain  after  the  observatory 
is  completed  The  income  from  this  sum  must  support 
the  observatory  for  the  present.  Although  the  whole  plan 
of  the  observatory  has  been  made  with  direct  reference  to 
keeping  its  running  expenses  low,  it  is  clear  that  the  com- 
pany of  astronomers  will  have  to  be  kept  small.  The  work 
of  these  observers  must  be  concentrated  on  the  large  equa- 
torial, and  even  then  their  energies  will  not  be  sufficient  to 
utilize  every  moment.  It  is  not  our  intention  to  jealously 
guard  the  immense  scientific  opportunity  for  ourselves,  for 
California,  or  even  for  the  United  States.  The  real  gift 
of  Mr.  Lick  was  to  the  world.  We  mean  to  put  the  large 
telescope  at  the  disposition  of  the  world,  by  inviting  its 
most  distinguished  astronomers  to  visit  us  one  at  a  time, 
and  to  give  them  the  use  of  the  instrument  during  certain 
specific  hours  of  the  twenty-four.  Each  day  there  will  be 
certain  hours  set  apart  when  the  observatory  staff  will  re- 
linquish the  use  of  the  equatorial  to  distinguished  special- 
ists who  will  come  upon  our  invitation  from  the  United 


THE  LICK  OBSERVATORY, 
Photogravure  from  a  photograph. 


a  small  fra< 

ray  < 
and  * 
the 


d,  but  it  will  faithfully 

t  falls  upon  it,  even  for  hours, 

its  automatic  register,  so  that 

that  this  can  be  measured,  if 

The  permanence  of  the  records 

:e,  and,  so  far  as  we  know,  it 

ii  plates  are  employed. 

:cessors  a  picture  of  the 

.ve  not  give  for  such 

.r>  or  by  Galileo! 

;iere  how  far  the 

it  it  to  engage  in 

.  that  the 

both 


-ervatory  was  $700,000, 

n  after  the  observatory 

sum  must  support 

whole  plan 

ork 

large  equa- 

be  sufficient  to 

T  intention  to  jealously 

ry  for  ourselves,  for 

The  real  gift 

>  mean  to  put  the  large 

by  inviting  its 

t  us  one  at  a  time, 

and  to  give  .ment  during  cer 

Each  day  there 

rs  set  apart  when  the  observatory  staff 
the  use  of  the  equatorial  to  distinguished 
>me  upon  our  invitation  from 


PHOTOGRAPHY  THE   SERVANT  OF  ASTRONOMY   307 

States  and  from  Europe  to  solve  or  to  attack  some  one  of 
the  many  unsolved  problems  of  astronomy.  In  this  way 
we  hope  to  make  the  gift  of  Mr.  Lick  one  which  is  truly 
a  gift  to  science,  and  not  merely  a  gift  to  California  and 
to  its  university. 

Even  under  such  circumstances  it  will  be  impossible 
to  utilize  the  instrumental  outfit  to  the  full.  It  was  clearly 
the  duty  of  the  Lick  trustees  to  make  this  observatory 
perfect  in  every  respect,  and  to  provide  it  with  all  the  in- 
struments necessary  to  a  complete  equipment.  This  they 
have  done  economically  and  wisely.  So  far  as  I  can  judge, 
there  is  nothing  that  should  be  altered.  The  instruments 
are  all  necessary,  and  they  are  mounted  in  the  most  perfect 
manner. 

Each  one  is  directly  subordinate  to  the  large  equatorial 
and  accessory  to  it.  Nothing  has  been  purchased,  and  no 
work  has  been  done,  which  does  not  directly  tend  to  make 
the  observations  made  by  the  large  equatorial  either  more 
complete  or  more  immediately  useful.  The  cost  of  the 
whole  observatory  may  fairly  be  said  to  be  the  cost  of 
the  great  telescope  in  place  and  entirely  ready  for  work. 
The  objective  itself  has  cost  $52,000.  The  photographic 
lens  will  add  $13,000  to  this.  The  mounting  which  is  to 
carry  the  tube  of  nearly  60  feet  in  length  is  to  be  made 
and  delivered  for  $42,000.  The  dome,  of  70  feet  interior 
diameter,  will  be  built  in  San  Francisco,  and  I  have  no 
doubt  that  it  will  be  materially  better  than  any  now  made. 
The  chief  novelty  will  be  the  adoption  of  Mr.  Grubb's  in- 
genious plan  for  placing  the  observer  in  a  proper  position 
with  reference  to  his  telescope.  We  have  to  recollect  that 
the  eyepiece  of  the  telescope  may  be  about  5  feet  from 
the  floor  of  the  dome  when  the  telescope  is  pointed  to  the 
zenith,  or  it  may  be  35  feet  when  the  telescope  is  in  the 
horizontal  position.  The  ordinary  observing  chair,  which 
is  convenient  enough  when  it  is  not  more  than  16  feet  high, 
becomes  a  cumbrous  and  inconvenient  affair  when  it  is 
extended  to  35  feet.  Mr.  Grubb  proposed  to  remedy  this 
by  raising  the  whole  floor  of  the  dome,  like  an  elevator, 


308  HOLDEN 

to  the  proper  height.  The  whole  floor  will  be  raised  ver- 
tically a  distance  of  i6J  feet  by  four  hydraulic  jacks.  The 
ascent  is  made  in  eight  minutes  with  a  perfectly  parallel 
motion.  The  water  supply  for  this  purpose  comes  from  the 
watershed  of  the  dome  itself.  The  last  mechanical  diffi- 
culty is  now  overcome,  and  it  is  expected  that  the  steel 
dome  will  be  mounted  during  the  present  year,  or  at  least 
in  the  spring  of  1887.  The  contract  price  for  the  dome 
delivered  and  erected  is  $56,800,  and  for  the  moving  floor 
$14,250.  The  sum  of  these  items  is  $178,000,  and  if  this 
is  increased  by  others  not  named  here  it  will  raise  the  cost 
of  the  large  instrument  in  place  to  $200,000.  The  prepara- 
tion of  the  top  of  the  mountain  to  receive  the  buildings,  the 
erection  of  the  buildings  themselves  and  the  observers* 
houses,  and  above  all  the  provision  of  an  adequate  water 
supply,  have  been  covered  by  the  remaining  $300,000. 

With  faithfulness  on  the  part  of  the  company  of  astron- 
omers to  which  this  magnificent  equipment  is  confided, 
and  with  the  generous  support  of  the  friends  of  science  in 
California,  much  may  be  expected  to  follow  from  this 
splendid  gift  to  America  and  to  the  world. 


THE   BEGINNINGS   OF  AMERICAN 
ASTRONOMY1    . 

IT  is  impossible,  even  in  the  briefest  sketch,  not  to  em- 
phasize the  debt  of  American  science  and  learning  to 
the  intelligent  interest  and  patronage  of  our  early  Presi- 
dents— Washington,  John  Adams,  Jefferson,  Madison, 
Monroe,  John  Quincy  Adams.  The  powerful  impetus 
given  by  them  and  through  them  has  shaped  the  liberal 
policy  of  our  Governments,  national  and  State,  toward  edu- 
cation and  toward  science.  Sir  Lyon  Playfair,  in  his  ad- 
dress to  the  British  Association  for  the  Advancement  of 
Science  (1885),  has  recognised  this  influence  in  the  truest 
and  most  graceful  way.  He  said:  "  In  the  United  King- 
dom we  are  just  beginning  to  understand  the  wisdom  of 
Washington's  '  Farewell  Address  to  his  Countrymen ' 
(1796)  when  he  said:  'Promote,  as  an  object  of  primary 
importance,  institutions  for  the  increase  and  diffusion  of 
knowledge;  in  proportion  as  the  structure  of  a  govern- 
ment gives  force  to  public  opinion,  it  is  essential  that  public 
opinion  should  be  enlightened/  ' 

Until  the  Revolution  (1776)  American  science  was  but 
English  science  transplanted,  and  it  looked  to  the  Royal 
Society  of  London  as  its  censor  and  patron.  Winthrop, 
Franklin,  and  Rittenhouse  were,  more  or  less,  English 
astronomers.  Franklin  was  the  sturdiest  American  of  the 
three.  As  early  as  1743  he  suggested  the  formation  of  the 
American  Philosophical  Society  of  Philadelphia.  John 
Adams  founded  the  American  Academy  of  Arts  and  Sci- 
ences in  Boston  in  1780.  These  two  societies,  together 

1  From  "  Science,"  June  18,  1897. 
309 


310  HOLDEN 

with  Harvard  College  (founded  in  1636),  Yale  College 
(1701),  the  University  of  Virginia  (founded  by  Jefferson  in 
1825),  and  the  United  States  Military  Academy  at  West 
Point  (1801),  were  the  chief  foci  from  which  the  light  of 
learning  spread.  Other  colleges  were  formed  or  forming 
all  over  the  Eastern  and  Middle  States  during  the  early 
years  of  the  century. 

The  leading  school  of  pure  science  was  the  Military 
Academy  at  West  Point,  and  it  continued  to  hold  this  place 
until  the  civil  war  of  1861.  From  its  corps  of  professors 
and  students  it  gave  two  chiefs  to  the  United  States  Coast 
Survey;  and  the  army,  particularly  the  corps  of  engineers, 
provided  many  observers  to  that  scientific  establishment, 
besides  furnishing  a  large  number  of  professors  and  teachers 
of  science  to  the  colleges  of  the  country.  The  observatory 
of  the  academy  was  founded  by  Bartlett  in  1841,  and  much 
work  was  done  there,  only  a  small  part  of  which  is  pub- 
lished. The  Coast  Survey  was  a  school  of  practice  for 
army  officers,  and  their  experience  was  utilized  in  numer- 
ous boundary  surveys  during  the  period  1830— '50.  Colonel 
J.  D.  Graham,  for  example,  v/as  astronomer  of  the  survey 
of  the  boundary  between  Texas  and  the  United  States  in 
1839-^40;  commissioner  of  the  Northeast  boundary  survey, 
1840-^43;  astronomer  of  the  Northwest  boundary  survey, 
1843-^47;  of  the  boundary  between  the  United  States  and 
Canada,  1848-^50;  of  the  survey  of  the  boundary  between 
Pennsylvania  and  Virginia,  1849-^50;  of  the  boundary 
survey  between  Mexico  and  the  United  States,  i85O-'5i. 
The  names  of  Bonneville,  Talcott,  Cram,  Emory,  and  other 
army  officers  are  familiar  in  this  connection,  and  their 
work  was  generally  of  a  high  order.  It  was  in  such  service 
that  Talcott  invented  or  re-invented  the  Zenith  Telescope, 
now  universally  employed  for  all  delicate  determinations  of 
latitude.  The  mechanical  tact  of  Americans  has  served 
astronomy  well.  The  sextant  was  invented  by  Thomas 
Godfrey,  of  Philadelphia,  in  1730,  a  year  before  Hadley 
brought  forward  his  proposal  for  such  an  instrument.2  The 
*  In  1700  Sir  Isaac  Newton  sent  drawings  and  descriptions  of  a 


THE   BEGINNINGS   OF   AMERICAN   ASTRONOMY     311 

chronograph  of  the  Bonds,  the  Zenith  Telescope  of  Talcott, 
and  the  break-circuit  chronometer  of  Winlock  are  univer- 
sally used  to-day  The  diffraction  gratings  of  Rutherfurd 
were  the  best  to  be  had  in  the  world  till  they  were  replaced 
by  those  of  Rowland.  The  use  of  a  telescope  as  a  collimator 
was  first  proposed  by  Rittenhouse.  The  pioneer  opticians 
of  the  United  States  were  Holcomb  (1826),  Fitz  (1846  or 
earlier),  Clark  (1845),  Spencer  (1851).  Only  the  Clarks 
have  a  world-wide  reputation.  Wurdemann,  instrument 
maker  to  the  United  States  Coast  Survey  (1834),  had  a 
decided  influence  on  observers  and  instrument  makers 
throughout  the  United  States,  as  he  introduced  extreme 
German  methods  and  models  among  us  where  extreme 
English  methods  had  previously  prevailed.  The  system  of 
rectangular  land  surveys  which  proved  to  be  so  convenient 
for  the  public  lands  east  of  the  Rocky  Mountains  was  de- 
vised and  executed  by  Mansfield,  a  graduate  of  the  Mili- 
tary Academy. 

The  list  of  army  officers  who  became  distinguished  in 
civil  life  as  professors  in  the  colleges  of  the  country  is  a 
very  long  one.  Courtenay  (class  of  1821  at  West  Point) 
was  professor  of  mathematics  at  the  University  of  Penn- 
sylvania, 1 834-^36,  at  the  University  of  Virginia,  1842-^43, 
and  was  the  author  of  admirable  text-books.  Norton  (class 
of  1831)  became  professor  at  New  Haven,  and  wrote -a 
very  useful  text-book  of  astronomy  in  1839;  and  the  list 
could  be  much  extended.  The  excellent  training  in  mathe- 
matics at  West  Point  (chiefly  in  French  methods)  early 
made  itself  felt  throughout  the  whole  country.  The  mathe- 
matical text-books  of  Peirce,  of  Harvard,  and  of  Chauvenet, 
of  the  Naval  Academy,  brought  the  latest  learning  of  Eu- 
rope to  American  students.  Mitchel  (class  of  1829  at  West 
Point)  was  the  only  graduate  who  became  a  professional 

reflecting  sextant  to  Halley  for  his  advice.  At  Halley's  death  these 
were  found  among  his  papers.  Hadley's  device  (1731)  was  undoubtedly 
derived  from  Newton's  manuscripts.  The  Royal  Society  of  London 
granted  two  hundred  pounds  to  Godfrey  for  his  invention,  which  his 
brother,  Captain  Godfrey,  had  previously  put  into  practical  use  in  the 
West  Indies. 


3i2  HOLDEN 

astronomer  (i842-'6i).  His  direct  service  to  practical  ob- 
serving astronomy  is  small,  but  his  lectures  (i842-'48),  the 
conduct  of  the  Cincinnati  Observatory  (i845~'59),  and  his 
publication  of  the  "Sidereal  Messenger"  (i846-'48),  to- 
gether with  his  popular  books,  excited  an  intense  and  wide- 
spread public  interest  in  the  science,  and  indirectly  led  to 
the  foundation  of  many  observatories.  He  was  early  con- 
cerned in  the  matter  of  using  the  electric  current  for  longi- 
tude determinations,  and  his  apparatus  was  only  displaced 
because  of  the  superior  excellence  of  the  chronograph  de- 
vised by  the  Bonds.  His  work  was  done  under  immense 
disadvantages,  in  a  new  community  (Ohio),  but  the  endow- 
ment of  astronomical  research  in  America  owes  a  large 
debt  to  his  energy  and  efforts. 

The  navy  and  the  United  States  Naval  Academy 
(founded  by  Bancroft  in  1845,  at  the  suggestion  of  Chau- 
venet)  were  very  active  in  astronomical  work.  Chauvenet 
(Yale  College,  1840)  published  a  text-book  of  trigonom- 
etry, in  1850,  which  had  an  important  share  in  directing 
attention  to  rigid,  elegant,  and  general  methods  of  research. 
His  "Astronomy"  (1863)  is  a  handbook  for  all  students. 
Walker,  Gilliss,  Coffin,  Hubbard,  Ferguson,  Keith,  Yarnall, 
Winlock,  Maury,  Wilkes,  were  all  connected  with  the 
navy  more  or  less  intimately.  Walker's  career  was  espe- 
cially brilliant;  he  graduated  at  Harvard  College  in  1825, 
and  established  the  observatory  of  the  Philadelphia  High 
School  in  1840.  He  was  the  leading  spirit  in  the  United 
States  Naval  Observatory  at  Washington  (1845-^47), 
and  introduced  modern  methods  into  its  practice  at  the 
beginning.  From  the  observatory  he  went  to  the  Coast 
Survey  to  take  charge  of  its  longitude  operations,  and  he 
continued  to  direct  and  expand  this  department  until  his 
death,  in  1853.  To  him,  more  than  to  any  single  person, 
is  due  the  idea  of  the  telegraphic  method  ("  the  American 
method  ")  of  determining  differences  of  longitude.  His 
assistant  in  this  work  was  Gould,  who  succeeded  to  the 
charge  of  it  in  1853.  His  researches  extended  to  the  field 
of  mathematical  astronomy  also,  and  his  theory  of  the 


THE   BEGINNINGS  OF   AMERICAN   ASTRONOMY 


313 


planet  Neptune  (then  newly  discovered)  marks  an  impor- 
tant step  forward.  His  investigations  and  those  of  Peirce 
were  conducted  in  concert,  and  attracted  general  and  de- 
served attention. 

The  exploring  expedition  of  Wilkes  required  corre- 
sponding observations  to  be  made  in  America,  and  during 
the  period  1838-^42  William  Bond,  at  Dorchester,  and 
Lieutenant  Gilliss,  at  Washington,  maintained  such  a  series 
with  infinite  assiduity  and  with  success.  The  results  of  Gil- 
liss's  astronomical  expedition  to  the  southern  hemisphere 
(Chile,  1 849-^52)  were  most  creditable  to  him  and  to  the 
navy,  though  his  immediate  object — the  determination  of 
the  solar  parallax — was  not  attained. 

The  Coast  Survey  began  its  work  in  1817  under  Hass- 
ler,  a  professor  from  West  Point,  who  impressed  upon  the 
establishment  a  thoroughly  scientific  direction.  Bache,  his 
successor  (a  grandson  of  Benjamin  Franklin),  was  a  gradu- 
ate of  West  Point  in  the  class  of  1825,  and  took  charge  of 
the  Survey  in  1843.  He  is  the  true  father  of  the  institu- 
tion, and  gave  it  the  practical  efficiency  and  high  standard 
which  characterized  its  work.  He  called  around  him  the 
flower  of  the  army  and  navy,  and  was  ably  seconded  by 
th^  permanent  corps  of  civilian  assistants — Walker,  Sax- 
ton,  Gould,  Dean,  Blunt,  Pourtales,  Boutelle,  Hilgard, 
Schott,  Goodfellow,  Cutts,  Davidson,  and  others. 

Silliman's  (and  Dana's)  "  American  Journal  of  Science  " 
had  been  founded  at  New  Haven  in  1818,  and  served  as  a 
medium  of  communication  among  scientific  men.  A  great 
step  forward  was  made  in  the  establishment  of  the  "  Astro- 
nomical Journal"  by  Dr.  Gould  on  his  return  from  Europe 
at  the  close  of  i849.3  Silliman's  "Journal"  was  chiefly  con- 
cerned with  the  non-mathematical  sciences,  though  it  has 
always  contained  valuable  papers  on  mathematics,  astron- 
omy, and  physics,  especially  from  the  observers  of  Yale 
College — Olmsted,  Herrick,  Bradley,  Twining,  Norton, 
Newton,  Lyman,  and  others.  In  Mason,  who  died  in  1840 

*  The  "  Astronomische  Nachrichten  "  had  been  founded  in  Altona, 
by  Schumacher,  in  1821. 


314 


HOLDEN 


at  the  age  of  twenty-one,  the  country  lost  a  practical  astron- 
omer of  the  highest  promise.4  Gould's  "  Journal  "  was  an 
organ  devoted  to  a  special  science.  It  not  only  gave  a  con- 
venient means  of  prompt  publication,  but  it  immediately 
quickened  research  and  helped  to  enforce  standards  already 
established  and  to  form  new  ones.  The  "  Astronomical  No- 
tices "  of  Briinnow  (1858-' 62)  might  have  been  an  ex- 
ceedingly useful  journal  with  an  editor  who  was  wil- 
ling to  give  more  attention  to  details,  but,  in  spite  of 
Briinnow's  charming  personality  and  great  ability,  it 
had  comparatively  little  influence  on  the  progress  of 
the  science. 

The  translation  of  the  "  Mecanique  Celeste  "  of  Laplace 
by  Nathaniel  Bowditch,  the  supercargo  of  a  Boston  ship 
(1815— '17),  marks  the  beginning  of  an  independent  mathe- 
matical school  in  America.  The  first  volume  of  the  trans- 
lation appeared  in  1829;  at  that  time  there  were  not  more 
than  two  or  three  persons  in  the  country  who  could  read 
it  critically.  The  works  of  the  great  mathematicians  and 
astronomers  of  France  and  Germany — Laplace,  Lagrange, 
Legendre,  Olbers,  Gauss,  W.  Struve,  Bessel — were  almost 
entirely  unknown. 

Bowditch's  translation  of  the  "  Mecanique  Celeste," 
and,  still  more,  his  extended  commentary,  brought  this 
monumental  work  to  the  attention  of  students  and  within 
their  grasp.  His  "  Practical  Navigator  "  5  contained  the 
latest  and  best  methods  for  determining  the  position  of  a 
ship  at  sea,  expressed  in  simple  rules.  American  navigators 
had  no  superiors  in  the  first  half  of  this  century.  Nan- 
tucket  whalers  covered  the  Pacific,  Salem  ships  swarmed 
in  the  Indies,  and  the  clipper  ships  made  passages  round 
the  Horn  to  San  Francisco,  which  are  a  wonder  to-day. 
Part  of  their  success  is  due  to  the  bold  enterprise  of  their 

4  See  the  "  International  Review,"  vol.  x,  p.  585. 

8  First  edition,  1802.  Sumner's  method  in  navigation  (1843) — a  very 
original  and  valuable  contribution  from  a  Boston  sea-captain — and 
Maury's  "  Wind  and  Current  Charts,"  begun  in  1844,  are  two  other 
notable  contributions  from  a  young  country  to  an  art  as  old  as  com- 
merce. 


THE   BEGINNINGS  OF  AMERICAN   ASTRONOMY     315 

captains  (who  were  said  to  carry  deck-loads  of  studding-sail 
booms  to  replace  those  carried  away),  but  an  important 
part  depended  on  their  skill  as  observers  with  the  sextant. 
One  of  the  sister  ships  to  the  one  of  which  Bowditch  was 
supercargo  was  visited  at  Genoa  by  a  European  astronomer 
of  note  (Baron  de  Zach),  who  found  that  the  latest  methods 
of  working  lunar  distances  to  determine  the  longitude  were 
known  to  all  on  board,  sailors  as  well  as  officers.  His  be- 
wilderment reached  its  climax  when  the  navigator  called 
the  negro  cook  from  the  galley  and  bade  him  expound  the 
methods  of  determining  the  longitude  to  the  distinguished 
visitor. 

On  Bowditch's  own  ship  there  was  "  a  crew  of  twelve 
men,  every  one  of  whom  could  take  and  work  a  lunar  ob- 
servation as  well,  for  all  practical  purposes,  as  Sir  Isaac 
Newton  himself."  Such  crews  were  only  to  be  found  on 
American  ships  in  the  palmy  days  of  democracy.  All  were 
cousins  or  neighbours,  and  each  had  a  "  venture  "  in  the 
voyage.  But  these  anecdotes  may  serve  as  illustrations  of 
the  intellectual  awakening  which  came  about  as  soon  as 
our  young  country  was  relieved  from  the  pressure  of  the 
two  wars  of  1776  and  1812.  An  early  visitor,  Baron  Hyde 
de  Neuville  (1805),  felt  "  an  unknown  something  in  the 
air,"  "  a  new  wind  blowing."  This  new  spirit,  born  of  free- 
dom, entered  first  into  practical  life,  as  was  but  natural; 
science  next  felt  its  impulse,  and,  last  of  all,  literature  was 
born.  Emerson  hailed  it  (in  1837)  "  as  the  sign  of  an  in- 
destructible instinct."  "  Perhaps  the  time  is  already  come," 
he  says,  "  when  the  sluggard  intellect  of  this  country  will 
look  from  under  its  iron  lids  and  fill  the  postponed  expecta- 
tion of  the  world  with  something  better  than  the  exertions 
of  mechanical  skill.  Our  day  of  dependence,  our  long  ap- 
prenticeship to  the  learning  of  other  lands,  draws  to  a  close. 
The  millions  that  around  us  are  rushing  into  life  can  not 
always  be  fed  with  the  sere  remains  of  foreign  harvests." 

Benjamin  Peirce,  a  graduate  of  Harvard  in  the  class  of 
1829,  had  been  concerned  with  the  translation  of  the 
"  Mecanique  Celeste,"  and  was  early  familiar  with  the  best 


316  HOLDEN 

mathematical  thought  of  Europe.  He  became  professor  in 
Harvard  College  in  1833,  and,  after  the  death  of  Bowditch, 
in  1838,  he  was  easily  the  first  mathematical  astronomer  in 
the  country.  His  instruction  was  precisely  fitted  to  de- 
velop superior  intelligences,  and  this  was  his  prime  useful- 
ness. Just  such  a  man  was  needed  at  that  time.  Besides 
his  theoretical  researches  on  the  orbits  of  the  planets  (espe- 
cially Uranus  and  Neptune)  and  of  the  moon,  his  study  of 
the  theory  of  perturbations,  and  his  works  on  pure  math- 
ematics and  mechanics,  he  concerned  himself  with  ques- 
tions of  practical  astronomy,  although  the  observations 
upon  which  he  depended  were  the  work  of  others.  He  was 
the  consulting  astronomer  of  the  "  American  Ephemeris 
and  Nautical  Almanac  "  from  its  foundation  in  1849,  an^ 
its  plans  were  shaped  by  him  to  an  important  degree.  His 
relative,  Lieutenant  Davis,  United  States  Navy  (the  trans- 
lator of  Gauss's  "  Theoria  Motus  Corporum  Ccelestium  ") 
(1857),  was  placed  in  charge  of  the  "  Ephemeris,"  and  the 
members  of  its  staff — Runkle,  Ferrel,  Wright,  Newcomb, 
Winlock,  and  others — most  effectively  spread  its  exact 
methods  by  example  and  precept.  Professor  Peirce  under- 
took the  calculations  relating  to  the  sun,  Mars,  and  Uranus 
in  the  early  volumes  of  the  "  Ephemeris."  As  a  compli- 
ment to  her  sex,  Miss  Maria  Mitchell  was  charged  with 
those  of  Venus,  Mercury  was  computed  by  Winlock, 
Jupiter  by  Kendall,  Saturn  by  Downes,  Neptune  by  Sears 
Walker. 

The  Smithsonian  Institution  was  founded  in  1846,  and 
Joseph  Henry  was  called  from  Princeton  College  to  direct 
it.  There  never  was  a  wiser  choice.  His  term  of  service 
(1846-^78)  was  so  long  that  his  ideals  became  firmly  fixed 
within  the  establishment,  and  were  impressed  upon  his  con- 
temporaries and  upon  a  host  of  younger  men.  The  inter- 
ests of  astronomy  were  served  by  the  encouragement  of 
original  research  through  subsidies  and  otherwise,  by  the 
purchase  of  instruments  for  scientific  expeditions,  by  the 
free  exchange  of  scientific  books  between  America  and 
Europe,  and  by  the  publication  of  the  results  of  recondite 


THE   BEGINNINGS  OF  AMERICAN   ASTRONOMY    317 

investigations.  It  is  by  these  and  like  services  that  the 
institution  is  known  and  valued  among  the  wide  commu- 
nity of  scientific  men  throughout  the  world. 

But  this  enumeration  of  specific  benefits  does  not  con- 
vey an  adequate  idea  of  the  immense  influence  exercised 
by  the  institution  upon  the  scientific  ideals  of  the  coun- 
try. It  was  of  the  first  importance  that  the  beginnings  of 
independent  investigation  among  Americans  should  be 
directed  toward  right  ends  and  by  high  and  unselfish  aims. 
In  the  formation  of  a  scientific  and,  as  it  were,  a  moral 
standard  a  few  names  will  ever  be  remembered  among  us; 
and  no  one  will  stand  higher  than  that  of  Henry.  His  wise, 
broad,  and  generous  policy,  and  his  high  personal  ideals, 
were  of  immense  service  to  his  colleagues  and  to  the 
country. 

The  establishment  of  a  national  observatory  in  Wash- 
ington was  proposed  by  John  Quincy  Adams  in  1825;  but 
it  was  not  until  1844  tnat  tne  United  States  Naval  Observa- 
tory was  built  by  Lieutenant  Gilliss,  of  the  navy,  from 
plans  which  he  had  prepared.  By  what  seems  to  have  been 
an  injustice  Gilliss  was  not  appointed  to  be  its  first  di- 
rector.6 This  place  fell  to  Lieutenant  M.  F.  Maury.  Gilliss 
had  been  on  detached  service  for  some  years,  and  a  rigid 
construction  of  rules  required  that  he  should  be  sent  to 
sea,  and  not  remain  to  launch  the  institution  which  he  had 
built  and  equipped. 

The  first  corps  of  observers  at  Washington  (1845)  con- 
tained men  of  first-class  ability — Walker,  Hubbard,  Coffin. 
Gilliss's  work  as  astronomer  to  the  Wilkes  Exploring  Expe- 
dition (1838-^42),  at  his  little  observatory  on  Capitol  Hill, 
had  shown  him  to  be  one  of  the  best  of  observers,  as  well 
as  one  of  the  most  assiduous.  His  study  and  experience 
in  planning  and  building  the  Naval  Observatory  had  broad- 
ened his  mind.  To  the  men  just  named,  with  Peirce, 
Gould,  and  Chauvenet,  and  to  their  coadjutors  and  pupils, 
we  owe  the  introduction  of  the  methods  of  Gauss,  Bessel, 
and  Struve  into  the  United  States,  and  it  is  for  this  reason 
8  He  was,  however,  director  during  the  years  1861-65. 


318  HOLDEN 

that  American  astronomy  is  the  child  of  German  and  not 
of  English  science. 

The  most  natural  evolution  might  seem  to  have  been 
for  Americans  to  follow  the  English  practice  of  Maskelyne 
and  Pond.  But  the  break  caused  by  the  War  of  Independ- 
ence, by  the  War  of  1812,  and  by  the  years  necessary  for 
our  youthful  governments  to  consolidate  (1776-1836), 
allowed  our  young  men  of  science  to  make  a  perfectly  un- 
biased choice  of  masters.  The  elder  Bond  (William  Cranch 
Bond,  born  1789,  director  of  Harvard  College  Observatory, 
i84O-'59)  was  one  of  the  older  school  and  received  his 
impetus  from  British  sources  during  a  visit  to  England 
in  1815. 

In  estimating  the  place  of  the  elder  Bond  among  sci- 
entific men  it  is  necessary  to  take  into  account  the  circum- 
stances which  surrounded  him.  He  was  born  in  the  first 
year  of  the  French  Revolution  (1789);  he  was  absolutely 
self-taught;  practically  no  astronomical  work  was  done  in 
America  before  1838.  While  Admiral  Wilkes  was  seeking 
for  coadjutors  to  prosecute  observations  in  the  United 
States  during  the  absence  of  his  exploring  expedition  he 
was  indeed  fortunate  in  finding  two  such  men  as  Bond  and 
Gilliss.  Their  assiduity  was  beyond  praise,  and  it  led  each 
of  them  to  important  duties.  Bond  became  the  founder 
and  director  of  the  Observatory  of  Harvard  College,  while 
Gilliss  is  the  father  of  the  United  States  Naval  Observatory 
at  Washington,  as  well  as  of  that  of  Santiago  de  Chile,  the 
oldest  observatory  in  South  America.  Cambridge,  though 
the  seat  of  the  most  ancient  university  in  America,  was  but 
a  village  in  1839.  The  college  could  afford  no  salary  to 
Bond,  but  only  the  distinction  of  a  title,  "  Astronomical 
Observer  to  the  University,"  and  the  occupancy  of  the 
Dana  house,  in  which  his  first  observatory  was  established. 
His  work  there,  as  elsewhere,  was  well  and  faithfully  done, 
and  it  led  the  college  authorities  to  employ  him  as  the 
astronomer  of  the  splendid  observatory  which  was  opened 
for  work  in  1847.  At  that  time  the  two  largest  telescopes 
in  the  world  were  those  of  the  Imperial  Observatory  of 


THE   BEGINNINGS   OF   AMERICAN   ASTRONOMY 


319 


Russia  (Pultowa)  and  its  companion  at  Cambridge.  Each 
of  these  instruments  has  a  long  and  honourable  history. 
Their  work  has  been  very  different.  Who  shall  say  that 
one  has  surpassed  the  other?  We  owe  to  Bond  and  his 
son  the  discovery  of  an  eighth  satellite  to  Saturn,  of  the 
dusky  ring  to  that  planet,  the  introduction  of  stellar  pho- 
tography, the  invention  of  the  chronograph,  by  which  the 
electric  current  is  employed  in  the  registry  of  observations, 
the  conduct  of  several  chronometric  expeditions  between 
Liverpool  and  Boston  to  determine  the  transatlantic  longi- 
tude, and  a  host  of  minor  discoveries  and  observations. 

Gilliss  visited  France  for  study  in  1835,  before  he  took 
up  his  duties  at  Washington.  The  text-books  of  Bond  and 
Gilliss  were  the  "  Astronomies  "  of  Vince  (1797-1808)  and 
of  Pearson  (1824-^29).  The  younger  Bond  (George  Phil- 
lips Bond,  born  1825;  Harvard  College,  1844;  director  of 
the  Harvard  College  Observatory,  1859-^65)  and  his  con- 
temporaries, on  the  other  hand,  were  firmly  grounded  in 
the  German  methods,  then,  as  now,  the  most  philosophical 
and  thorough. 

It  was  not  until  1850,  or  later,  that  it  was  indispensable 
for  an  American  astronomer  to  read  the  German  language 
and  to  make  use  of  the  memoirs  of  Bessel,  Encke,  and 
Struve  and  the  text-books  of  Sawitsch  and  Brunnow.7 
This  general  acquaintance  with  the  German  language  and 
methods  came  nearly  a  generation  later  in  England.  The 
traditions  of  Piazzi  and  Oriani  came  to  America  with  the 
Jesuit  Fathers  of  Georgetown  College  (1844),  of  whom 
Secchi  and  Sestini  are  the  best  known. 

The  dates  of  the  foundation  of  a  few  observatories  of 
the  United  States  may  be  set  down  here.  Those  utilized 
for  the  observation  of  the  transit  of  Venus  in  1769  were 
temporary  stations  merely.  The  first  college  observatory 
was  that  of  Chapel  Hill,  North  Carolina  (1831);  Williams 
College  followed  (1836);  Hudson  Observatory  (Ohio) 
(1838);  the  Philadelphia  High  School  (1840);  the  Dana 

T  Dr.   Bowditch  learned  to   read  German   in   1818,  at  the  age  of 
forty-five. 


320  HOLDEN 

House  Observatory  of  Harvard  College  (1840);  West 
Point  (1841);  the  United  States  Naval  Observatory  (1844); 
the  Georgetown  College  Observatory  (1844);  the  Cincin- 
nati Observatory  (1845);  tne  new  observatory  of  Harvard 
College  (1846);  the  private  observatory  of  Dr.  Lewis  M. 
Rutherfurd  in  New  York  city  (1848);  the  observatory  at 
Ann  Arbor  (1854);  the  Dudley  Observatory  at  Albany 
(1856);  and  that  of  Hamilton  College  (1856). 

These  dates  and  the  summary  history  just  given  will 
serve  to  indicate  the  situation  of  astronomy  in  the  United 
States  during  the  first  half  of  the  present  century.  A  little 
attention  to  the  dates  will  enable  the  reader  to  place  an  indi- 
vidual or  an  institution  on  its  proper  background.  It  must 
constantly  be  kept  in  mind  that  the  whole  country  was  very 
young,  and  that  public  interest  in  astronomical  matters 
was  neither  educated  nor  very  general.  The  data  here  set 
down  will  have  a  distinct  value  as  a  contribution  to  the 
history  of  astronomy  in  America.  The  developments  of 
later  years  have  been  so  amazing  that  we  forget  that  the 
first  working  observatories  were  founded  so  late  as  1845. 

American  science  is  scarcely  more  than  half  a  century 
old.  The  day  will  soon  come — it  is  now  here — when  we 
shall  look  back  with  wonder  and  gratitude  to  ask  who  were 
the  men  who  laid  the  wide  and  deep  foundations  which 
already  maintain  so  noble  an  edifice. 


STELLAR   PARALLAX 

BY 

SIR   JOHN   FREDERICK  WILLIAM   HERSCHEL 

(Born  1792;  died  1871) 


21 


STELLAR  PARALLAX1 

GENTLEMEN:  The  report  of  the  council  has  placed 
before  you  so  ample  a  view  of  the  state  of  the  so- 
ciety, of  its  labours  during  the  last  year,  of  the  ac- 
cessions of  its  members,  and  of  the  many  and  severe  losses 
it  has  had  to  deplore,  that  little  is  left  for  me  to  add,  except 
my  congratulations  on  its  continued  and  increasing  pros- 
perity. It  would  be  inexpressibly  gratifying  to  me  if  I 
could  persuade  myself  that  my  own  exertions  in  its  chair 
had  contributed,  even  in  a  small  degree,  to  that  prosperity ; 
but,  alas!  I  have  felt  only  too  sensibly  how  very  feebly  and 
inefficiently,  especially  during  the  last  year,  owing  to  a 
variety  of  causes,  but  chiefly  to  residence  at  a  distance  from 
London,  I  have  been  able  to  fill  that  most  honourable 
office. 

The  immediate  object  of  my  now  addressing  you,  gen- 
tlemen, is  to  declare  the  award  by  your  council  of  the  gold 
medal  of  this  society  to  our  eminent  associate,  M.  Bessel, 
for  his  researches  on  the  annual  parallax  of  that  remarkable 
double  star  61  Cygni — researches  which  it  is  the  opinion 
of  your  council  have  gone  so  far  to  establish  the  existence 
and  to  measure  the  quantity  of  a  periodical  fluctuation, 
annual  in  its  period  and  identical  in  its  law  with  parallax, 
as  to  leave  no  reasonable  ground  for  doubt  as  to  the  reality 
of  such  fluctuation,  as  something  different  from  mere  in- 
strumental or  observational  error:  an  inequality,  in  short, 
which,  if  it  is  not  parallax,  is  so  inseparably  mixed  up  with 

1  Address  on  presenting  the  gold  medal  of  the  Royal  Astronomical 
Society  to  Prof.  F.  W.  Bessel,  Director  of  the  Observatory  of  Konigs- 
berg.  (From  "  Monthly  Notices  of  the  Royal  Astronomical  Society," 
1841.) 

323 


324  HERSCHEL 

that  effect  as  to  leave  us  without  any  criterion  by  which  to 
distinguish  them.  Now,  in  such  a  case,  parallax  stands  to 
us  in  the  nature  of  a  vera  causa,  and  the  rules  of  philoso- 
phizing will  not  justify  us  in  referring  the  observed  effect 
to  an  unknown  and,  so  far  as  we  can  see,  an  inconceivable 
cause,  when  this  is  at  hand  ready  to  account  for  the  whole 
effect. 

I  say  in  the  nature  of  a  vera  causa,  since  each  particu- 
lar star  must  of  necessity  be  of  some  parallax.  Every  real, 
existing  material  body  must  enjoy  that  indefeasible  attri- 
bute of  body — viz.,  definite  place.  Now,  place  is  defined 
by  direction  and  distance  from  a  fixed  point.  Every  body, 
therefore,  which  does  exist,  exists  at  a  certain  definite  dis- 
tance from  us  and  at  no  other,  either  more  or  less.  The  dis- 
tance of  every  individual  body  in  the  universe  from  us 
is,  therefore,  necessarily  admitted  to  be  finite. 

But  though  the  distance  of  each  particular  star  be  not 
in  strictness  infinite,  it  is  yet  a  real  and  immense  accession 
to  our  knowledge  to  have  measured  it  in  any  one  case.  To 
accomplish  this  has  been  the  object  of  every  astronomer's 
highest  aspirations  ever  since  sidereal  astronomy  acquired 
any  degree  of  precision.  But  hitherto  it  has  been  an  object 
which,  like  the  fleeting  fires  that  dazzle  and  mislead  the 
benighted  wanderer,  has  seemed  to  suffer  the  semblance 
of  an  approach  only  to  elude  his  seizure  when  apparently 
just  within  his  grasp,  continually  hovering  just  beyond  the 
limits  of  his  distinct  apprehension,  and  so  leading  him  on 
in  hopeless,  endless,  and  exhausting  pursuit. 

The  pursuit,  however,  though  eager  and  laborious,  has 
been  far  from  unproductive,  even  in  those  stages  where  its 
immediate  object  has  been  baffled. 

The  fact  of  a  periodical  fluctuation  of  some  kind  in  the 
apparent  places  of  the  stars  was  recognised  by  Flamsteed, 
and  erroneously  attributed  to  parallax.  The  nearer  exami- 
nation of  this  phenomenon  with  far  more  delicate  instru- 
ments, infinitely  greater  refinement  of  methods,  and  clearer 
views  of  the  geometrical  relations  of  the  subject,  rewarded 
Bradley  with  his  grand  discoveries  of  aberration  and  nuta- 


STELLAR   PARALLAX  325 

tion,  and  enabled  him  to  restrict  the  amount  of  possible 
parallax  of  the  stars  observed  by  him  within  extremely  nar- 
row limits. 

Bradley  failed  to  detect  any  appreciable  parallax,  though 
he  considered  i"  as  an  amount  which  would  not  have 
escaped  his  notice.  And  since  his  time  this  quantity  has 
been  assumed  as  a  kind  of  conventional  limit,  which  it 
might  be  expected  to  attain,  but  hardly  to  surpass.  But 
this  was  rather  because,  in  the  best  observations  from 
Bradley's  time  forward,  i"  has  been  a  tolerated  error;  a 
quantity  for  which  observation  and  mechanism,  joined  to 
atmospheric  fluctuations  and  uncertainties  of  reduction, 
could  not  be  held  rigidly  accountable  even  in  mean  results, 
than  from  any  reason  in  the  nature  of  the  case,  or  any 
distinct  perception  of  its  reality.  If  parallax  were  to  be 
detected  at  all  by  observations  of  the  absolute  places  of  the 
stars,  it  could  only  emerge  as  a  "  residual  phenomenon," 
after  clearing  away  all  the  effects  of  the  uranographical  cor- 
rections as  well  as  of  refraction,  when  it  would  remain 
mixed  up  with  whatever  uncertainties  might  remain  as  to 
the  coefficients  of  the  former,  with  the  casual  irregularities 
of  the  latter,  and  with  all  the  forms  of  instrumental  and 
observational  error.  Now,  these  have  hitherto  proved  suf- 
ficient, even  in  the  observation  of  zenith  stars,  quite  to 
overlay  and  conceal  that  minute  quantity  of  which  astron- 
omers were  in  search. 

It  is  not  my  intention,  gentlemen,  to  enter  minutely 
into  the  history  of  the  attempts  of  various  astronomers  on 
this  problem,  whether  by  the  discussion  of  observations 
of  one  star,  or  by  the  combination  of  those  of  pairs  of  stars 
opposite  in  right  ascension;  nor  with  the  occasional  gleams 
of  apparent  success  which,  however,  have  always  proved 
illusory,  which  have  attended  these  attempts.  For  such  a 
history,  and,  indeed,  for  a  complete  and  admirably  drawn- 
up  monograph  of  the  whole  subject,  I  must  refer  to  a  paper 
lately  read  to  this  society  by  Mr.  Main,  and  which  is  now 
in  process  of  publication  in  the  forthcoming  volume  of  our 
"  Memoirs."  In  whatever  reference  I  may  have  to  make 


326  HERSCHEL 

to  the  history  of  the  subject,  I  must  take  this  opportunity 
to  acknowledge  my  obligations  to  the  author  of  this  paper, 
as  well  as  for  his  exceedingly  luminous  exposition  of  the 
results  of  those  more  successful  attempts  on  the  problem 
by  Henderson,  Struve,  and  Bessel,  which  I  shall  now  pro- 
ceed more  especially  to  consider. 

It  would  be  wrong,  however,  not  to  notice  that  the  first 
indication  of  some  degree  of  impression  beginning  to  be 
made  on  the  problem  seems  to  be  found  in  Struve's  dis- 
cussion of  the  differences  of  right  ascension  of  circumpolar 
stars  in  1819,  1820,  and  1821.  The  only  positive  result, 
indeed,  of  these  observations  is  that,  in  the  case  of  twenty- 
seven  stars  examined,  none  has  a  parallax  amounting  to 
half  a  second.  But  below  this  there  certainly  do  seem  to 
be  indications  in  the  nature  of  a  real  parallax,  which  might 
at  least  suffice  to  raise  the  sinking  hopes  of  astronomers 
and  excite  them  to  further  efforts. 

But  the  time  arrived  when  the  problem  was  to  be  at- 
tacked from  a  quarter  offering  the  greater  advantages,  and 
exposed  to  few  or  none  of  those  unmanageable  sources  of 
irregular  error  to  which  the  determination  of  absolute 
places  are  liable.  I  mean  by  the  measurement  of  the  dis- 
tances of  such  double  stars  as  consist  of  individuals  so  dif- 
ferent in  magnitude  as  to  authorize  a  belief  of  their  being 
placed  at  very  different  distances  from  the  eye;  or,  as 
Struve  expresses  it,  optically  and  not  physically  double. 
This,  in  fact,  was  the  original  notion  which  led  to  the 
micrometrical  measurements  of  double  stars;  but  not  only 
was  anything  like  a  fair  trial  of  the  method  precluded  by 
the  imperfections  of  all  the  micrometers  in  use  until  re- 
cently, but  the  interesting  phenomena  of  another  kind, 
which  began  to  unfold  themselves  in  the  progress  of  those 
measurements,  led  attention  off  altogether  from  this  their 
original  application,  which  thus  lay  dormant  and  neglected, 
until  the  capital  modern  improvements,  both  in  the  optical 
and  mechanical  parts  of  refracting  telescopes,  and  the  great 
precision  which  it  was  found  practicable,  by  their  aid,  to 
attain  in  these  delicate  measurements,  revived  the  idea  of 


STELLAR   PARALLAX  327 

giving  this  method,  what  it  never  before  had,  a  fair  trial. 
The  principle  on  which  the  determination  of  parallax  by 
means  of  micrometrical  observations  of  a  double  star  turns, 
is  extremely  simple.  If  we  conceive  two  stars  very  nearly 
in  a  line  with  the  eye,  but  of  which  one  is  vastly  more  re- 
mote than  the  other,  each,  by  the  effect  of  parallax,  will 
appear  to  describe  annually  a  small  ellipse  about  the  mean 
place  as  its  centre.  These  two  ellipses,  however,  though 
similar  in  form,  will  differ  in  dimension,  that  described  by 
the  more  remote  star  being  comparatively  much  smaller; 
consequently,  the  apparent  places  being  similarly  situated 
in  each,  their  apparent  distance  on  the  line  joining  these 
apparent  places  will  both  oscillate  in  angular  position  and 
fluctuate  in  length,  thus  giving  rise  to  an  annual  relative 
alternate  movement  between  the  individuals  both  in  posi- 
tion and  distance,  which  is  greater  the  greater  the  differ- 
ence of  the  parallaxes. 

Thus  it  is  not  the  absolute  parallax  of  either,  but  the 
differences  of  their  parallaxes,  which  is  effectively  measured 
by  this  method — i.  e.,  by  repeating  the  measurements  of 
their  mutual  distance  at  all  times  of  the  year.  But,  on  the 
other  hand,  aberration,  nutation,  precession,  and  refrac- 
tion act  equally  on  both  stars,  or  so  very  nearly  so  as 
to  leave  only  an  exceedingly  small  fraction  of  these  cor- 
rections bearing  on  the  results.  And  when  the  stars  are 
very  unequal  in  magnitude,  there  is  a  presumption  that  the 
difference  of  their  parallaxes  is  very  nearly  equal  to  the 
whole  parallax  of  the  nearer  one. 

The  selection  of  a  star  for  observation  involves  many 
considerations.  In  that  pitched  on  by  M.  Bessel  (61 
Cygni)  the  large  star  so  designated  is,  in  fact,  a  fine  double 
star;  nay,  one  that  has  been  ascertained  to  be  physically 
double.  It  is  in  every  respect  a  highly  remarkable  star. 
The  mutual  distance  of  its  individuals  is  great,  being  about 
i6y .  Now,  this  being  necessarily  less  than  the  axis  of  their 
mutual  orbit,  affords  in  itself  a  presumption  that  the  star 
is  a  near  one.  And  this  presumption  is  increased  by  the 
unusually  great  proper  motion  of  this  binary  system,  which 


328 


HERSCHEL 


amounts  to  nearly  5"  per  annum,  and  which  has  beeri 
made  by  Sir  James  South  the  subject  of  particular  inquiry, 
and  found  to  be  not  participated  in  by  several  small  sur- 
rounding stars,  which,  therefore,  are  not  physically  con- 
nected with  it.  Moreover,  the  angular  rotation  of  the  two, 
one  about  the  other,  has  been  well  ascertained. 

Now,  it  fortunately  happens  that  of  these  small  sur- 
rounding stars  there  are  two  very  advantageously  situated 
for  micrometrical  comparison  with  either  of  the  individuals 
of  the  binary  star,  or  with  the  middle  point  between  them. 
The  one  of  these  (a),  at  a  distance  of  f  42",  is  situated 
nearly  at  right  angles  to  the  direction  of  the  double  star; 
the  other  (b),  at  a  distance  of  n'  46",  nearly  in  that  direc- 
tion. Considering  a  and  I  as  fixed  points,  then,  and 
measuring  at  any  instant  of  time  their  distances  from  c, 
the  middle  point  of  the  double  star,  the  situation  of  c 
relative  to  a  and  b  is  ascertained;  and  if  this  be  done  at 
every  instant,  the  relative  locus  of  c,  or  the  curve  described 
by  it  on  the  plane  of  the  heavens  with  respect  to  the  fixed 
base  line  a  b,  will  become  known. 

Now,  on  the  hypothesis  of  parallax,  that  locus  ought 
to  be  an  ellipse  of  one  certain  calculable  eccentricity  and 
no  other,  and  its  major  and  minor  axes  ought  to  hold  with 
respect  to  the  points,  a,  b,  certain  calculable  positions  and 
no  other.  Hence  it  follows  that  the  distance  a  c  and  b  c  will 
each  of  them  be  subject  to  annual  increase  and  diminution; 
and  that,  first,  in  a  given  and  calculable  ratio  the  one  to  the 
other;  and,  secondly,  so  that  the  maxima  and  minima  of 
the  one  distance  a  c  shall  be  nearly  contemporaneous  with 
the  mean  values  of  the  other  distance  (b  c),  and  vice  versa. 

Thus  we  have,  in  the  first  place,  several  particulars  in- 
dependent of  mere  numerical  magnitudes;  and,  in  the  sec- 
ond place,  several  distinct  relations  a  priori  determined,  to 
which  those  numerical  values  must  conform,  if  it  be  true 
that  any  observed  fluctuations  in  these  distances  (a  c,  b  c) 
be  really  parallactic.  So  that  if  they  be  found  in  such  con- 
formity, and  the  above-mentioned  maxima  and  minima  do 
observe  that  interchangeable  law  above  stated;  and  if, 


STELLAR  PARALLAX  329 

moreover,  all  due  care  be  proved  to  have  been  taken  to 
eliminate  every  instrumental  source  of  annual  fluctuation — 
there  becomes  accumulated  a  body  of  probability  in  favour 
of  the  resulting  parallax  which  can  not  but  impress  every 
reasonable  mind  with  a  strong  degree  of  belief  and  con- 
viction. 

Now,  all  these  circumstances  have  been  found  by  M. 
Bessel,  in  his  discussion  of  the  measures  taken  by  him 
(which  have  been  very  carefully  and  rigorously  examined 
by  Mr.  Main  in  the  paper  alluded  to,  as  have  also  M.  Bes- 
sel's  formulae  and  calculations,  for  in  such  matters  nothing 
must  remain  unverified),  to  prevail  in  a  very  signal  and 
satisfactory  manner.  Not  one  case  of  discordance,  in  so 
many  independent  particulars,  has  been  found  to  subsist; 
and  this,  of  itself,  is  high  ground  of  probability.  But  we 
may  go  much  further.  Mr.  Main  has  projected  graphically 
the  deviations  of  the  distances  a  c  and  b  c  from  their 
mean  quantities  (after  clearing  them  of  the  effects  of 
proper  motion  and  of  the  minute  differences  of  aberration, 
etc.).  Taking  the  time  for  an  abscissa,  and  laying  down 
the  deviations  in  the  distances  so  cleared  as  ordinates,  two 
curves  are  obtained,  the  one  for  the  star  a,  the  other  for 
the  star  b.  Each  of  these  curves  ought  alternately  to  lie 
for  half  a  year  above  and  for  half  a  year  below  its  axis. 
It  does  so.  Each  of  them  ought  to  intersect  its  axis  at 
those  dates  when  the  maximum  and  minimum  of  the  other 
above  and  below  the  axis  occurs.  With  only  a  slight  de- 
gree of  hesitation  at  one  crossing,  it  does  so.  The  points 
of  intersection  with  the  axis  ought  to  occur  at  dates  in  like 
manner  calculated  a  priori;  and  so  they  do  within  very 
negligible  limits  of  error.  And,  lastly,  the  general  forms, 
magnitudes,  and  fixtures  of  the  curves  ought  to  be  identical 
with  those  of  curves  similarly  projected  by  calculation  on 
an  assumed  resulting  parallactic  coefficient.  This  is  the 
final  and  severe  test;  Mr.  Main  has  applied  it,  and  the  re- 
sults have  been  placed  before  you:  oculis  subjecta  fidelibus. 
If  all  this  does  not  carry  conviction  along  with  it,  it  seems 
difficult  to  say  what  ought  to  do  so. 


330  HERSCHEL 

The  only  thing  that  can  possibly  be  cavilled  at  is  the 
shortness  of  the  period  embraced  by  the  observations — viz., 
from  August,  1837,  to  the  end  of  March,  1840.  But  this 
interval  admits  of  five  intersections  of  each  curve  with  its 
axis;  of  two  maxima  and  two  minima  in  its  excursions  on 
either  side;  and  of  ample  room  for  trying  its  agreement  in 
general  form  with  the  true  parallactic  curves.  Under  such 
circumstances  it  is  quite  out  of  the  question  to  declare  the 
whole  phenomenon  an  accident  or  an  illusion.  Something 
has  assuredly  been  discovered,  and  if  that  something  be  not 
parallax,  we  are  altogether  at  fault,  and  know  not  what 
other  cause  to  ascribe  it  to. 

The  instrument  with  which  Bessel  made  these  most  re- 
markable observations  is  a  heliometer  of  large  dimensions, 
and  with  an  exquisite  object-glass  by  Fraunhofer.  I  well 
remember  to  have  seen  this  object-glass  at  Munich  before 
it  was  cut,  and  to  have  been  not  a  little  amazed  at  the 
boldness  of  the  maker  who  would  devote  a  glass,  which  at 
that  time  would  have  been  considered  in  England  almost 
invaluable,  to  so  hazardous  an  operation.  Little  did  I  then 
imagine  the  noble  purpose  it  was  destined  to  accomplish. 
By  the  nature  and  construction  of  this  instrument,  espe- 
cially when  driven  by  clockwork,  almost  every  conceivable 
error  which  can  effect  a  micrometrical  measure  is  destroyed, 
when  properly  used;  and  the  precautions  taken  by  M. 
Bessel  in  its  use  have  been  such  as  might  be  expected  from 
his  consummate  skill.  The  only  possible  apparent  opening 
for  an  annually  fluctuating  error  seems  to  be  in  the  cor- 
rection for  temperature  of  its  scale.  But  this  correction 
has  been  ascertained  by  M.  Bessel  by  direct  observation, 
in  hot  and  cold  seasons,  and  applied.  Nor  could  this  cause 
destroy  the  evidence  arising  from  the  simultaneous  observa- 
tion of  the  two  companion  stars,  since  a  wrong  correction 
for  temperature  would  affect  both  their  distances  propor- 
tionally, leaving  the  apparent  parallactic  movement  still 
unaccounted  for. 

The  resulting  parallax  is  an  extremely  minute  quantity, 
only  -f^f  of  a  second,  which  would  place  the  star  in  ques- 


STELLAR   PARALLAX  331 

tion  at  a  distance  from  us  of  nearly  670,000  times  that  of 
the  sun!  2  Such  is  the  universe  in  which  we  exist,  and 
which  we  have  at  length  found  the  means  to  subject  to 
measurement,  at  least  in  one  of  its  members,  probably 
nearer  to  us  than  the  rest. 

It  becomes  necessary  for  me  now  to  refer  to  two  series 
of  researches  on  this  important  subject,  which  have  been 
held  by  your  council  to  merit  very  high  and  honourable 
mention;  though  neither  of  them,  separately,  for  reasons 
which  I  shall  state,  would  have  been  considered  as  carrying 
that  weight  of  probability  in  favour  of  its  conclusions,  which 
would  justify  any  immediate  decision  of  the  nature  which 
they  have  come  to  in  the  case  of  M.  Bessel's.  I  allude  to 
M.  Struve's  inquiries,  by  the  method  of  micrometric  meas- 
ures, into  the  parallax  of  a  Lyrae,  and  to  Mr.  Henderson's, 
by  that  of  meridian  observations,  on  the  parallax  of  a 
Centauri. 

a  Lyrse  is  accompanied  by  a  very  minute  star,  at  the 
distance  of  about  43".  That  this  star  is  unconnected  with  a 
by  any  physical  relation  is  clear  from  the  fact,  ascertained 
by  Sir  James  South  and  myself,  that  it  does  not  participate 
in  the  proper  motion  of  the  large  star.  The  mutual  angular 
distance  of  these  stars  has  been  made  by  M.  Struve  the  sub- 
ject of  a  very  extensive  series  of  micrometric  measures  with 
the  celebrated  Dorpat  achromatic,  bearing  this  object 
steadily  in  view,  and  working  it  out  to  a  conclusion  of  the 
very  same  kind,  and  though  materially  inferior  in  the  degree 
and  nature  of  its  evidence  to  that  of  Bessel,  yet  certainly 
entitled  to  high  consideration.  M.  Struve's  observations  on 
this  star,  and  for  this  purpose,  extend  from  November, 
1835,  to  August,  1838,  and  are  distributed  over  sixty 
nights,  averaging  twenty  per  annum;  and  from  their  com- 
bination, according  to  the  principle  of  probabilities,  he  con- 
cludes a  parallax  of  0.261".  Mr.  Main  has  subjected  these 
observations  to  an  analysis  and  graphical  projection  pre- 

2  The  orbit  described  by  the  two  stars  of  61  Cygni  about  each  other 
will,  therefore,  be  about  fifty  times  the  diameter  of  the  earth's  about 
the  sun,  or  two  and  a  half  times  that  of  Uranus. 


332 


HERSCHEL 


cisely  similar  in  principle  to  those  I  have  explained  in  the 
case  of  61  Cygni.  The  curves  so  projected  have  been  sub- 
jected to  your  inspection,  and  that  inspection  certainly  does 
leave  a  very  strong  impression  of  a  real  and  tolerably  well- 
ascertained  parallax  having  been  detected  in  this  star.  But 
at  the  same  time  an  impression  no  less  decided,  owing  to 
irregularities  in  the  march  of  the  curve,  when  compared 
with  the  true  parallactic  curve,  is  created :  that  the  errors  of 
observation  are  far  from  being  eliminated;  that,  on  the  con- 
trary, they  bear  such  a  proportion  to  the  parallax  itself  as 
to  leave  room  for  some  degree  of  hesitation,  and  to  justify 
an  appeal  to  a  longer  series  of  observations,  and  to  con- 
current evidence  from  other  quarters,  before  declaring  any 
positive  opinion.  The  evidence  of  this  kind,  in  short,  is  not 
equal  to  that  afforded  by  the  similar  projection  of  Bessel's 
observations  of  either  of  his  two  comparison  stars.  And  to 
this  it  must  be  added  that  only  one  star  of  comparison  ex- 
isting in  the  line  of  a  Lyrae,  the  possible  effect  of  tem- 
perature and  annual  instrumental  variation  is  not  elimi- 
nated from  the  result  in  the  way  in  which  it  is  from  the 
measures  of  61  Cygni;  while  all  that  great  mutual  support 
which  the  observations  of  parallaxes  of  the  two  comparison 
stars  afford  each  other  in  the  latter  case  is  altogether  want- 
ing in  the  former.  These  considerations,  without  any 
underestimation  of  the  great  importance  and  value  of  M. 
Struve's  researches,  yet  formed  essential  drawbacks  on  the 
immediate  admission  of  his  results. 

In  a  word,  I  conceive  the  question  of  discovery  as  be- 
tween these  illustrious  but  most  generous  and  amicable 
rivals  may  be  thus  fairly  stated.  M.  Struve's  meridian  ob- 
servations in  1819-^2 1  seem  to  have  made  the  first  im- 
pression on  the  general  problem,  but  too  slight  to  authorize 
more  than  a  hope  that  it  would  yield  at  no  distant  day. 
His  micrometric  measures  of  a  Lyrse  commenced  more 
than  a  year  earlier,  and  have  extended  altogether  over  a 
longer  period  than  M.  Bessel's  of  61  Cygni.  From  their 
commencement  they  afford  indications  of  parallax,  and 
these  indications  accumulating  with  time  have  amounted  to 


STELLAR   PARALLAX 


333 


a  high  degree  of  probability,  and  rendered  the  supposition 
of  parallax  more  admissible  than  that  of  instrumental  or 
casual  errors  producing  the  same  influence  on  the  measures. 
On  the  other  hand,  M.  Bessel's  measures,  commencing  a 
year  later  and  continued  on  the  whole  through  somewhat 
less  time,  have  exhibited  a  compact  and  consistent  body  of 
evidence  drawn  from  two  distinct  systems  of  measures  mu- 
tually supporting  each  other,  and  so  steadily  bearing  on 
their  object  as  to  leave  no  more  reasonable  doubt  of  the 
truth  than  in  the  case  of  many  things  which  we  look  upon 
as,  humanly  speaking,  certain.  And  this  conviction,  once 
obtained,  reacts  on  our  belief  in  the  other  results,  and  in- 
duces us  to  receive  and  admit  it  on  the  evidence  adduced 
for  it;  which,  without  such  conviction  so  obtained,  we 
might  hesitate  to  do  until  after  longer  corroboration  of  the 
same  kind. 

The  other  series  of  observations  to  wrhich  I  must  now 
call  your  attention  are  those  of  Mr.  Henderson,  made  at 
the  Cape  of  Good  Hope,  on  the  great  star  a  Centauri,  the 
third  star  in  brightness  which  the  heavens  offer  to  our 
view.  It  is  a  magnificent  double  star  consisting  of  two 
individuals,  the  one  of  a  high  and  somewhat  brownish 
orange,  the  other  of  a  fine  yellow  colour,  and  each  of  which 
I  consider  fairly  entitled  to  be  classed  in  the  first  magni- 
tude.3 Their  distance  is  at  present  about  15"  asunder,  but 
it  is  rapidly  diminishing,  and  in  no  great  lapse  of  time  they 
will  probably  occult  one  another,  their  angular  motion  be- 
ing comparatively  small.  Their  apparent  distance  was  for- 
merly much  greater — how  much  we  can  not  say  for  want 
of  observations,  but  probably  the  major  axis  of  their  mutual 
orbit  is  little  short  of  a  minute  of  space.  They,  therefore, 
afford  strong  indications  of  being  very  near  our  system. 
Add  to  which  their  proper  motion  is  very  considerable,  and 
participated  in  by  both,  which  proves  their  connection  as 
a  binary  system;  and  an  additional  presumption  in  favour 

'I  have  seen  both  their  images  projected  on  a  screen  of  three 
thicknesses  of  stout  paper,  the  eye  being  on  the  opposite  side  of  the 
screen  from  that  on  which  the  images  were  depicted. 


334 


HERSCHEL 


of  their  proximity  may  be  drawn  from  their  situation  in 
what,  from  general  aspect,  I  gather  to  be  the  nearest  region 
of  the  Milky  Way,  among  an  immensity  of  large  stars. 

Mr.  Henderson  observed  these  stars  with  great  care 
both  in  right  ascension  and  declination  with  the  very  fine 
transit,  and  (in  spite  of  certain  grievous  defects  in  the  axis) 
the  otherwise  really  good  and  finely  divided  mural  circle  of 
the  Royal  Observatory  in  that  colony.  Since  his  return  to 
England  he  has  reduced  these  observations  with  a  view  to 
parallax,  and  the  result  is  the  apparent  existence  of  that 
element  to  what,  after  what  has  been  said,  we  must  now  call 
the  great  and  conspicuous  amount  of  a  full  second.  Mr. 
Main,  to  whom  I  am  so  largely  indebted  for  allowing  me 
to  draw  so  freely  on  his  labours,  has  also  discussed  these 
results,  and  comes  to  the  conclusion  that  (as  might,  per- 
haps, be  expected)  the  right  ascension  observations  afford 
a  trace,  but  an  equivocal  one,  of  parallax,  but  that  in  declina- 
tion (I  use  his  words)  "  the  law  of  parallax  is  followed  re- 
markably well.  There  is  scarcely  an  exception  to  the 
proper  change  of  sign,  according  to  the  change  of  sign  of 
the  coefficients  of  parallax.  This  is  quite  as  much  as  can 
reasonably  be  expected  in  a  series  of  individual  results  ob- 
tained from  any  meridional  instrument  for  observing  zenith 
distances.  We  can  not  expect  to  find  the  periodical  func- 
tion regularly  exhibited  by  the  differences.  On  the  whole, 
therefore,  we  should  say  that,  in  addition  to  the  claims  of 
a  Centauri  on  our  attention  with  relation  to  its  parallax, 
arising  from  its  forming  a  binary  system,  its  great  proper 
motion,  and  its  brightness,  it  derives  now  much  additional 
importance,  in  this  point  of  view,  from  the  investigation  of 
Mr.  Henderson.  This  we  are  at  least  entitled  to  assume 
until  some  distinct  reason,  independent  of  parallax,  shall 
have  been  assigned  for  the  changes  in  the  declinations. 
Such  I  do  not  consider  impossible,  having  before  my  eyes 
the  results  which  Dr.  Brinkley  derived,  in  the  cases  of  cer- 
tain stars,  from  the  Dublin  circle.  For  the  present  it  must 
be  considered  that  the  star  well  deserves  a  rigorous  exam- 
ination by  all  the  methods  which  the  author  himself  has  so 


STELLAR   PARALLAX  335 

well  pointed  out;  and  that,  in  the  event  of  a  parallax  at  all 
comparable  with  that  assigned  by  Mr.  Henderson  being 
found,  he  will  deserve  the  merit  of  its  first  discovery,  and 
the  warmest  thanks  of  astronomers,  as  an  extender  of  the 
knowledge  which  we  possess  of  our  connection  with  the 
sidereal  system." 

With  this  view  of  Mr.  Henderson's  labours  I  fully  agree, 
and  await  with  highly  excited  interest  the  result  of  Mr. 
Maclear's  larger  and  complete  series  of  observations  on  this 
star,  both  with  the  old  circle  and  that  more  perfect  one  with 
which  the  munificence  of  Government  has  recently  supplied 
the  observatory.  Should  a  different  eye  and  a  different 
circle  continue  to  give  the  same  results,  we  must,  of  course, 
acquiesce  in  the  conclusion,  and  the  distinct  and  entire 
merit  of  the  first  discovery  of  the  parallax  of  a  fixed  star 
will  rest  indisputably  with  Mr.  Henderson.  At  present, 
however,  we  should  not  be  justified  in  so  far  anticipating  a 
decision  which  time  alone  can  stamp  with  the  seal  of  abso- 
lute authenticity. 

Gentlemen  of  the  Astronomical  Society,  I  congratulate 
you  and  myself  that  we  have  lived  to  see  the  great  and 
hitherto  impassable  barrier  to  our  excursions  into  the  side- 
real universe — that  barrier  against  which  we  have  chafed  so 
long  and  so  vainly  (aestuantes  angusto  limite  mundi) — 
almost  simultaneously  overleaped  at  three  different  points. 
It  is  the  greatest  and  most  glorious  triumph  which  practical 
astronomy  has  ever  witnessed.  Perhaps  I  ought  not  to 
speak  so  strongly;  perhaps  I  should  hold  some  reserve  in 
favour  of  the  bare  possibility  that  it  may  be  all  an  illusion, 
and  that  further  researches,  as  they  have  repeatedly  before, 
so  may  now,  fail  to  substantiate  this  noble  result.  But  I 
confess  myself  unequal  to  such  prudence  under  such  excite- 
ment. Let  us  rather  accept  the  joyful  omens  of  the  time, 
and  trust  that,  as  the  barrier  has  begun  to  yield,  it  will 
speedily  be  effectually  prostrated.  Such  results  are  among 
the  fairest  flowers  of  civilization.  They  justify  the  vast  ex- 
penditure of  time  and  talent  which  have  led  up  to  them; 
they  justify  the  language  which  men  of  science  hold,  or 


336  HERSCHEL 

ought  to  hold,  when  they  appeal  to  the  governments  of 
their  respective  countries  for  the  liberal  devotion  of  the 
national  means  in  furtherance  of  the  great  objects  they  pro- 
pose to  accomplish.  They  enable  them  not  only  to  hold 
out,  but  to  redeem  their  promises,  when  they  profess  them- 
selves productive  labourers  in  a  higher  and  richer  field  than 
that  of  mere  material  and  physical  advantages.  It  is  then 
when  they  become  (if  I  may  venture  on  such  a  figure  with- 
out irreverence)  the  messengers  from  Heaven  to  earth  of 
such  stupendous  announcements  as  must  strike  every  one 
who  hears  them  with  almost  awful  admiration,  that  they 
may  claim  to  be  listened  to  when  they  repeat  in  every 
variety  of  urgent  instance  that  these  are  not  the  last  of  such 
announcements  which  they  shall  have  to  communicate; 
that  there  are  yet  behind,  to  search  out  and  declare,  not  only 
secrets  of  Nature  which  shall  increase  the  wealth  or  power 
of  man,  but  truths  which  shall  ennoble  the  age  and  coun- 
try in  which  they  are  divulged,  and,  by  dilating  the  intel- 
lect, react  on  the  moral  character  of  mankind.  Some  truths 
are  things  quite  as  worthy  of  struggles  and  sacrifices  as 
many  of  the  objects  for  which  nations  contend,  and  ex- 
haust their  physical  and  moral  energies  and  resources. 
They  are  gems  of  real  and  durable  glory  in  the  diadems  of 
princes,  and  conquests  which,  while  they  leave  no  tears 
behind  them,  continue  forever  unalienable. 

It  must  be  needless  for  me  to  express  a  hope  that  these 
researches  will  be  followed  up.  Already  we  have  to  con- 
gratulate astronomy  on  the  resolution  taken  by  one  of  our 
great  academic  institutions  to  furnish  its  observatory  with 
a  heliometer  of  the  same  description  as  Bessel's;  nor  can 
we  fear  but  that  the  research  will  speedily  be  extended  to 
other  stars,  offering  varieties  of  magnitude  and  other  indi- 
cations to  draw  attention  to  them. 

On  the  whole,  then,  the  award  of  our  medal,  which  the 
council  have  agreed  on,  seems  to  me,  under  the  circum- 
stances, fully  justified.  I  will  now  request  the  foreign  sec- 
retary to  convey  it  to  our  distinguished  associate;  and  in 
so  doing  I  will  add  our  hope  that,  in  the  painful  and  dis- 


STELLAR  PARALLAX  337 

tressing  visitation  with  which  it  has  pleased  Providence  re- 
cently to  try  him,  he  may  find  occasion  to  withdraw  his 
mind  awhile  from  that  melancholy  contemplation  to  re- 
ceive with  satisfaction  such  a  tribute  to  this  his  last  and 
perhaps  his  greatest  achievement,  accompanied  as  it  is  by 
the  truest  regard  for  his  private  worth  and  the  most  re- 
spectful sympathy  for  his  present  distress. 

"  If  we  ask  to  what  end  magnificent  establishments  are 
maintained  by  states  and  sovereigns,  furnished  with  master- 
pieces of  art,  and  placed  under  the  direction  of  men  of 
first-rate  talent  and  high-minded  enthusiasm,  sought  out 
for  those  qualities  among  the  foremost  in  the  ranks  of 
science,  if  we  demand  cui  bono?  for  what  good  a  Bradley 
has  toiled,  or  a  Maskelyne  or  a  Piazzi  has  worn  out  his 
venerable  age  in  watching,  the  answer  is,  not  to  settle 
mere  speculative  points  in  the  doctrine  of  the  universe;  not 
to  cater  for  the  pride  of  man  by  refined  inquiries  into  the 
remoter  mysteries  of  Nature;  not  to  trace  the  path  of  our 
system  through  space,  or  its  history  through  past  and  future 
eternities.  These,  indeed,  are  noble  ends,  and  which  I  am 
far  from  any  thought  of  depreciating;  the  mind  swells  in 
their  contemplation,  and  attains  in  their  pursuit  an  expan- 
sion and  a  hardihood  which  fit  it  for  the  boldest  enterprise. 
But  the  direct  practical  utility  of  such  labours  is  fully  wor- 
thy of  their  speculative  grandeur.  The  stars  are  the  land- 
marks of  the  universe;  and,  amid  the  endless  and  compli- 
cated fluctuations  of  our  system,  seem  placed  by  its  Creator 
as  guides  and  records,  not  merely  to  elevate  our  minds  by 
the  contemplation  of  what  is  vast,  but  to  teach  us  to  direct 
our  actions  by  reference  to  what  is  immutable  in  his  works. 
It  is,  indeed,  hardly  possible  to  over-appreciate  their  value 
in  this  point  of  view.  Every  well-determined  star,  from  the 
moment  its  place  is  registered,  becomes  to  the  astrono- 
mer, the  geographer,  the  navigator,  the  surveyor,  a  point 
of  departure  which  can  never  deceive  or  fail  him,  the  same 
forever  and  in  all  places,  of  a  delicacy  so  extreme  as  to  be 
a  test  for  every  instrument  invented  by  man,  yet  equally 

22 


338  HERSCHEL 

adapted  for  the  most  ordinary  purposes;  as  available  for 
regulating  a  town  clock  as  for  conducting  a  navy  to  the 
Indies;  as  effective  for  mapping  down  the  intricacies  of  a 
petty  barony  as  for  adjusting  the  boundaries  of  transatlan- 
tic empires.  When  once  its  place  has  been  thoroughly 
ascertained  and  carefully  recorded,  the  brazen  circle  with 
which  that  useful  work  was  done  may  moulder,  the  marble 
pillar  may  totter  on  its  base,  and  the  astronomer  himself 
survive  only  in  the  gratitude  of  posterity;  but  the  record 
remains,  and  transfuses  all  its  own  exactness  into  every  de- 
termination which  takes  it  for  a  groundwork,  giving  to 
inferior  instruments — nay,  even  to  temporary  contrivances, 
and  to  the  observations  of  a  few  weeks  or  days — all  the  pre- 
cision attained  originally  at  the  cost  of  so  much  time,  la- 
bour, and  expense." 


THE  HISTORY  OF  THE 
TELESCOPE 

BY 

CHARLES  S.   HASTINGS 


THE   HISTORY  OF  THE  TELESCOPE1 

THERE  is  no  instrument  which  has  done  so  much  to 
widen  the  scope  of  human  knowledge,  to  extend 
our  notions  of  the  universe,  and  to  stimulate  intel- 
lectual activity  as  has  the  telescope,  unless  the  microscope 
be  regarded  as  a  successful  rival.  But  even  admitting  a 
parity  in  scientific  importance,  the  former  instrument  is 
incomparably  more  interesting  in  its  history,  in  the  same 
degree  that  its  history  is  more  simple  and  more  compre- 
hensible. To  trace  its  development  from  a  curious  toy  in 
the  hands  of  its  discoverer,  for  we  shall  see  that  this  term 
is  more  appropriate  than  inventor,  to  the  middle  of  this 
century,  is  to  be  brought  into  contact  with  most  of  the 
great  philosophers,  from  the  time  of  the  Renaissance,  who 
have  achieved  greatness  in  physical  science,  Galileo,  Tor- 
ricelli,  Huygens,  Cassini,  Newton,  Halley,  Kepler,  Euler, 
Calaiault,  the  Herschels,  father  and  son,  Fraunhofer,  Gauss 
— from  only  a  portion  of  the  list  of  great  names.  Its 
growth  toward  perfection  has  constantly  carried  with  it 
increased  precision  in  the  applied  sciences  of  navigation  and 
of  all  branches  of  engineering.  It  would  be  easy  to  show 
that  even  pure  mathematics  would  be  in  a  far  less  forward 
state  had  there  been  no  problems  of  astronomy  and  physics 
which  were  first  suggested  by  the  employment  of  the  tele- 
scope. It  is  to  this  history  that  I  venture  to  invite  your 
attention  this  evening.  I  purpose  to  review  succinctly  the 
origin  and  development  of  this  potent  aid  in  the  study  of 

1  Address  delivered  at  the  dedication  of  the  Goodsell  Observatory  of 
Carleton  College,  Northfield,  Minn.,  June  n,  1891.  (From  the  "  Side- 
real Messenger,"  August,  1891,  vol.  x,  pp.  335-354.) 


342  HASTINGS 

Nature,  to  name  some  of  the  more  important  achievements 
depending  upon  it,  and  to  trace  its  gradual  improvement 
to  the  magnificent  and  complicated  instrument  which  con- 
stitutes the  modern  equatorial.  After  this  sketch  I  shall 
try  to  give  an  idea  of  the  imperfections  which  the  consci- 
entious artisan  has  to  contend  with  in  attaining  perfection, 
and  to  make  clear  the  methods  which  have  been  employed 
in  reducing  these  imperfections  in  the  noble  instrument 
now  erected  at  this  institution,2  and  explain  why  its  pos- 
sessors are  so  hopeful  of  gratifying  success. 

Galileo  learned,  in  1609,  while  visiting  Venice,  that  a 
marvellous  instrument  had  been  invented  the  preceding 
year  in  Holland,  which  would  enable  an  observer  to  see  a 
distant  object  with  the  same  distinctness  as  if  it  were  only 
at  a  small  fraction  of  its  real  distance.  It  required  but  little 
time  for  the  greatest  physicist  of  his  age  to  master  the  prob- 
lem thus  suggested  to  his  mind,  and  after  his  return  to 
Padua,  where  he  held  the  position  of  professor  of  mathe- 
matics in  the  famous  university  of  that  city,  he  set  himself 
earnestly  to  work  making  telescopes.  Such  was  his  success 
that  in  August  of  the  same  year  he  sent  to  the  Venetian 
senate  a  more  perfect  instrument  than  they  had  been  able 
to  procure  from  Holland ;  and  in  January  of  the  next  year, 
by  means  of  a  telescope  magnifying  thirty  times,  he  discov- 
ered the  four  satellites  of  Jupiter.  This  brilliant  discovery 
was  followed  by  that  of  the  mountains  in  the  moon ;  of  the 
variable  phases  of  Venus,  which  established  the  Copernican 
theory  of  the  solar  system  as  incontestable,  and  of  the  true 
nature  of  the  Milky  Way,  together  with  many  others  of  less 
philosophical  importance.  Though  Galileo  did  not  change 
the  character  of  the  telescope  as  it  was  known  to  its  dis- 
coverer in  Holland,  he  made  it  much  more  perfect;  and 
above  all,  made  the  first  and  most  fertile  application  of  the 
instrument  to  increase  the  bounds  of  human  knowledge, 
so  that  it  is  inevitable  that  his  name  should  be  indissolubly 
connected  with  the  instrument.  Thus  the  form  which  he 
used  is  to  this  day  known  as  the  Galilean  telescope. 
3  Carleton  College. 


THE   HISTORY  OF  THE   TELESCOPE  343 

Considering  the  enormous  interest  excited  throughout 
intellectual  Europe  by  the  invention  of  the  telescope,  it 
seems  surprising  that  its  early  history  is  so  confused.  Less 
than  two  years  after  it  was  first  heard  of,  a  discovery,  per- 
haps the  greatest  of  a  thousand  years  in  the  domain  of 
natural  philosophy,  had  been  made  by  its  means.  Notwith- 
standing these  facts,  the  three  contemporary,  or  nearly 
contemporary,  investigators  assign  the  honour  to  three 
different  persons,  and  if  we  should  write  out  the  names 
of  all  those  to  whom  more  modern  writers  have  attributed 
the  invention,  the  list  would  be  a  long  one.  The  surprise 
will  not  be  boundless,  however,  if  we  consider  the  task 
before  a  historian  in  the  next  century  who  undertakes 
to  justly  apportion  the  honour  of  the  invention  of  the 
telephone  among  its  numerous  claimants.  The  analogy, 
though  suggested  in  the  obvious  fact  that  the  telephone 
is  to  hearing  just  what  the  telescope  is  to  sight,  may  be 
made  much  closer  if  we  could  imagine  the  future  historian 
deprived  of  all  but  verbal  description,  that  contemporary 
diagrams  and  models  were  wholly  wanting.  Under  such 
conditions  it  is  difficult  to  believe  that  the  historian  would 
easily  escape  antedating  the  discovery  of  the  telephone 
proper  on  account  of  descriptions,  generally  imperfect,  of 
the  acoustic  telephone.  But  this  would  fairly  represent 
the  condition  of  the  material  at  the  command  of  an  inves- 
tigator of  the  present  day  into  a  question  of  science  of  the 
early  part  of  the  seventeenth  century.  No  wonder,  then, 
that  the  invention  has  been  attributed  to  Archimedes,  to 
Roger  Bacon,  to  Porta,  and  to  many  others  who  have 
written  on  optics;  but  to  find  the  name  of  Satan  in  the 
list  is  certainly  surprising.  Still  we  read  that  a  very  learned 
man  of  the  seventeenth  century,  named  Arias  Montanus, 
finds  in  the  fourth  chapter  of  Matthew,  eighth  verse,  evi- 
dence that  Satan  possessed,  and  probably  invented,  a  tele- 
scope; otherwise,  how  could  he  have  "shown  him  all  the 
kingdoms  of  the  world  and  the  glory  of  them  "?  3  It  seems 

8  The  history  of  the  telescope  is  admirably  treated  in  Poggendorff's 
"  Geschichte  der  Physik,"  from  which  the  statements  above  are  taken. 


344 


HASTINGS 


to  be  well  established  now,  however,  that  Franz  Lippershey, 
or  Lippersheim,  a  spectacle-maker  at  Middleberg,  was  the 
real  inventor  of  the  telescope,  and  that  Galileo's  first  tele- 
scope, avowedly  suggested  by  news  of  the  Hollander's 
achievement,  was  an  independent  invention. 

That  this  discovery  was  really  an  accident  we  may  be 
quite  sure,  for  not  only  was  there  no  developed  theory  of 
optics  at  that  time,  but  even  the  law  of  refraction,  which 
lies  at  the  basis  of  such  theory,  was  quite  unknown.  So, 
too,  it  seems  to  me  quite  certain  that  Galileo's  invention 
must  have  been  empirical  and  guided  by  somewhat  precise 
information,  such  as  that  the  instrument  consisted  essen- 
tially of  two  lenses,  of  which  one  was  a  magnifying  and  the 
other  a  diminishing  lens.  At  least,  that  Galileo's  telescope 
was  like  that  of  the  Hollander;  that,  theoretically  consid- 
ered, it  is  not  so  simple  as  that  made  of  two  magnifying 
lenses,  as  is  evinced  by  the  fact  that  Kepler,  the  first  phi- 
losopher to  establish  an  approximate  theory  of  optical 
instruments,  only  two  years  later  invented  the  latter  and 
prevailing  form;  and  finaly,  that  Galileo  published  no  con- 
tributions to  the  theory  of  optics,  seem  quite  sufficient  rea- 
sons for  such  a  belief.  But,  in  any  case,  Galileo's  merit  is 
in  no  wise  lessened  by  having  failed  to  do  what  could  not 
be  done  at  that  time,  and  the  value  of  his  discoveries  in 
emancipating  men's  minds  from  authority  in  matters  of 
pure  reason  is  incalculable. 

No  other  discoveries  of  great  moment  were  made  until 
over  a  generation  after  Galileo  proved  the  existence  of 
spots  on  the  sun  in  1611.  This  cessation  of  activity  was 
doubtless  owing  to  the  difficulty  of  securing  telescopes  of 
greater  efficiency  than  that  possessed  by  Galileo,  and  which 
he  would  hardly  have  left  until  its  powers  of  discovery  had 
been  fully  exhausted  in  his  own  hands.  By  the  middle  of 
the  seventeenth  century,  however,  several  makers  of  lenses 
had  so  far  improved  the  methods  of  grinding  and  polishing, 
that  telescopes  notably  superior  in  power  to  that  of  Galileo 
were  procurable.  Of  these  Torricelli,  Divini,  and  Campani, 
all  Italians;  Auzout,  who  constructed  a  telescope  six  hun- 


THE  HISTORY  OF  THE  TELESCOPE  345 

dred  feet  in  length,  though  no  means  was  ever  found  for 
directing  such  an  enormous  instrument  toward  the  heavens; 
but  above  all,  Huygens — have  won  distinction  as  telescope 
makers.  The  last-named  philosopher  discovered,  by  means 
of  a  telescope  of  his  construction,  the  largest  satellite  of 
Saturn  in  1655,  thus  adding  a  fifth  member  to  the  list  of 
planetary  bodies  unknown  to  the  ancients.  But  his  most 
important  astronomical  discovery,  made  also  in  1655,  was 
the  nature  of  the  rings  of  Saturn.  This  object  had  greatly 
puzzled  Galileo,  to  whose  small  telescope  the  planet  ap- 
peared to  consist  of  a  larger  sphere  flanked  on  either  side 
by  a  smaller  one;  but  when  in  the  course  of  the  orbital 
motion  of  Saturn  the  rings  entirely  disappeared  he  was 
wholly  unable  to  suggest  an  explanation.  This  planet  had 
thus  presented  a  remarkable  problem  to  all  astronomical 
observers  for  more  than  forty  years,  and  the  records  of  the 
efforts  to  solve  it  during  that  interval  afford  us  a  most  ex- 
cellent means  of  judging  the  progress  in  practical  optics. 
Huygens  announced  these  discoveries  early  in  1656,  but 
that  relating  to  the  ring  was  given  in  the  form  of  an  ana- 
gram, the  solution  of  which  was  first  published  in  1659. 
This  discovery  was  contested  in  Italy  by  Divini,  but  was 
finally  confirmed  by  members  of  the  Florentine  Academy 
with  one  of  Divini's  own  telescopes. 

A  few  years  later  the  famous  astronomer  Cassini,  having 
come  to  Paris  from  Italy  as  royal  astronomer,  commenced 
a  series  of  brilliant  discoveries  with  telescopes  made  by 
Campani,  of  Rome.  With  these,  varying  in  length  from 
35  feet  to  136  feet,  he  discovered  four  satellites  to  Saturn 
in  addition  to  the  one  discovered  by  Huygens.  The 
whole  number  was  increased  by  Herschel's  discovery  of 
two  smaller  ones  in  1789,  a  hundred  and  five  years  after 
Cassini's  last  discovery,  and  again  by  Bond's  discovery  of 
an  eighth  in  1848.  The  Saturnian  system,  to  which  the 
telescope  has  doubtless  been  directed  more  frequently  than 
to  anything  else,  thus  serves  as  a  record  of  the  successive 
improvements  of  the  telescope.  Highly  significant  is  the 
fact  that  the  discoveries  of  the  eighteenth  century  were 


346  HASTINGS 

made  with  a  reflecting  telescope,  the  others  all  being  with 
refracting  instruments. 

Cassini's  discovery  in  1684  of  the  two  satellites  now 
known  as  Tethys  and  Dione,  was  not  accepted  as  conclu- 
sive until  long  afterward,  when  Pound,  1718,  with  a  tele- 
scope 123  feet  in  length,  which  Huygens  had  made  and 
presented  to  the  Royal  Society,  saw  all  five.  This  particu- 
lar instrument  is  of  especial  interest,  because  it  is  the  only 
one  of  those  of  the  last  half  of  the  seventeenth  century 
which  has  been  carefully  compared  with  modern  instru- 
ments. Moreover,  it  is  without  doubt  quite  equal  in  merit 
to  any  of  that  period.  But  we  find  that,  although  it  had  a 
diameter  of  six  inches,  its  performance  was  hardly  better 
than  that  of  a  perfect  modern  telescope  of  four  inches  in 
diameter,  and,  perhaps,  four  feet  and  a  half  in  length,  while 
in  regard  to  convenience  in  use  the  modern  compact  in- 
strument is  incomparably  superior. 

Another  notable  discovery  of  this  period  was  that  of  the 
duplicity  of  the  rings  of  Saturn  by  the  Ball  brothers  in  1665, 
though  its  independent  discovery  by  Cassini  ten  years  later 
first  attracted  the  attention  of  astronomers.  The  earlier 
discovery  was  made  by  means  of  a  telescope  thirty-eight 
feet  long,  which  seems  to  have  been  of  English  manufac- 
ture. We  must  regard  Cassini's  discovery  of  the  third  and 
fourth  satellites  of  Saturn,  however,  as  marking  the  very 
farthest  reach  of  the  old  form  of  telescope;  a  century  was 
to  elapse  and  an  entirely  new  form  of  telescope  was  to  be 
developed  before  another  considerable  addition  to  our 
knowledge  of  the  aspect  of  the  heavenly  bodies  was  to  be 
made.  It  is  true  larger  telescopes  were  made,  and  Huygens 
invented  a  means  by  which  -they  could  be  used  without 
tubes,  but  notwithstanding  this  improvement  they  proved 
so  cumbersome  as  to  be  impracticable. 

The  older  opticians  had  found  that  if  they  attempted  to 
increase  the  diameter  of  a  telescope  they  were  obliged  to 
increase  its  length  in  a  much  more  rapid  ratio  to  secure  dis- 
tinct vision.  The  reason  of  this  was  not  clearly  understood, 
but  it  was  supposed  to  be  owing  to  the  fact  that  a  wave 


THE   HISTORY  OF  THE  TELESCOPE  347 

front,  changed  in  curvature  by  passing  through  a  spherical 
surface,  is  no  longer  strictly  spherical.  This  deviation  in 
shape  of  the  refracted  wave  from  a  true  sphere  is  called 
spherical  aberration.  When  the  refracting  surfaces  are  large 
and  of  considerable  curvature,  this  soon  becomes  very  seri- 
ous, but  by  using  small  curvature,  which,  in  a  telescope,  ob- 
viously corresponds  to  great  length,  the  effects  of  the  error 
can  be  made  insensible.  Newton's  discovery  of  the  com- 
posite nature  of  light  and  of  the  phenomenon  of  dispersion 
enabled  him  to  explain  the  true  cause  of  indistinctness  in 
short  telescopes — namely,  that  the  refraction  by  the  objec- 
tive varies  for  different  colours;  consequently,  if  the  ocular 
is  placed  for  one  particular  colour,  it  will  not  be  in  the  right 
position  for  any  of  the  others,  whence  the  image  of  a  star 
or  planet  will  seem  to  be  surrounded  by  a  fringe  of  coloured 
light.  Newton  found  this  source  of  indistinctness  in  the 
image,  which  is  now  known  as  chromatic  aberration,  many 
hundred  times  as  serious  as  the  spherical  aberrations.  As 
he  was  persuaded  by  his  experiments  that  this  obstacle 
to  further  improvement  in  the  refracting  telescope  was  in- 
superable, he  turned  his  attention  to  a  form  of  telescope 
which  had  been  suggested  a  number  of  years  earlier,  in 
which  the  image  was  to  be  formed  by  reflection  from  a 
concave  mirror,  and  constructed  a  small  one  with  his  own 
hands  which  is  still  in  the  possession  of  the  Royal  Society. 
This  little  instrument  seems  to  have  been  of  about  the 
same  power  as  Galileo's  instrument  with  which  he  discov- 
ered the  satellites  of  Jupiter,  but  it  was  hardly  more  than 
six  inches  in  length. 

Since  that  time  the  reflecting  telescope  has  had  a  re- 
markable history  of  development  in  the  hands  of  a  number 
of  most  skilful  mechanicians,  who  have  also  for  the  most 
part  been  distinguished  by  their  discoveries  in  physical 
astronomy;  we  may  therefore  advantageously  depart  from 
the  chronological  treatment  and  follow  the  history  of  this 
type  of  instrument.  This  course  is  the  more  natural  be- 
cause we  may  probably  regard  the  supremacy  of  the  re- 
flector (undisputed  a  century  ago)  as  passed  away  forever. 


348  HASTINGS 

Even  after  Newton's  invention  was  made  public,  little 
was  done  toward  the  improvement  of  telescopes  for  half 
a  century,  until  Hadley  presented  a  reflector  of  his  own 
construction  to  the  Royal  Society  in  1723,  which  was  found 
to  be  equal  to  the  Huygens  refractor  of  123  feet  in 
length.  From  this  time  we  may  date  the  beginning  of  the 
superiority  of  reflectors.  A  few  years  later  Short  com- 
menced his  career  as  a  practical  optician,  and  for  thirty 
years  he  was  unapproached  in  the  excellence  of  his  instru- 
ments. During  this  time  many  telescopes,  more  powerful 
than  the  best  of  the  previous  century  and  infinitely  more 
convenient  in  use,  had  been  made  and  scattered  throughout 
Europe,  but  during  this  period  also  there  was  a  singluar 
dearth  of  telescopic  discovery.  Perhaps  men  thought  that 
the  harvest  had  already  been  gathered ;  or,  perhaps,  we  may 
find  the  explanation  in  that  the  great  cost  of  telescopes  so 
restricted  their  use  that  the  impulse  to  discovery  by  their 
means  was  confined  to  a  very  small  class.  In  view  of  the 
remarkable  manner  in  which  the  standstill  in  this  branch 
of  science  was  finally  followed  by  a  brilliant  period  of  dis- 
covery, rivalled  alone  by  that  of  Galileo,  we  might  well  re- 
gard the  latter  cause  as  the  chief  one. 

William  Herschel  was  born  in  1738,  in  Hanover.  In 
1755  he  left  his  native  country,  and  going  to  England,  se- 
cured a  position  as  organist  in  Octagon  Chapel,  Bath, 
where  we  find  him  in  1766.  Here  he  became  so  profoundly 
interested  in  the  views  of  the  heavens  which  a  borrowed 
telescope  of  moderate  power  yielded,  that  he  tried  to  pur- 
chase one  in  London.  The  cost  of  a  satisfactory  instru- 
ment proving  beyond  his  command,  he  determined  to  con- 
struct one  with  his  own  hands.  Thus  he  entered  upon  a 
course  which  was  to  reflect  honour  upon  himself,  his  coun- 
try, and  his  age,  and  which  was  to  add  more  to  physical 
astronomy  than  any  other  one  man  has  added  before  or 
since.  With  almost  inconceivable  industry  and  persever- 
ance he  cast,  ground,  and  polished  more  than  four  hundred 
mirrors  for  telescopes,  varying  in  diameter  from  six  to 
forty-eight  inches.  This  in  itself  would  imply  a  busy  life 


THE   HISTORY  OF  THE  TELESCOPE 


349 


in  any  artisan,  but  when  we  remember  that  all  this  was 
merely  subsidiary  to  his  main  work  of  astronomical  dis- 
covery, we  can  not  withhold  our  admiration. 

Fortunately  for  science  as  well  as  for  himself,  he  made 
early  in  his  career  a  discovery  of  the  very  first  importance, 
which  attracted  the  attention  of  all  Christendom.  On  the 
night  of  March  13,  1781,  Herschel  was  examining  small 
stars  in  the  constellation  of  Gemini  with  one  of  his  tele- 
scopes of  a  little  more  than  six  inches  in  diameter,  when  he 
perceived  one  that  appeared  "  visibly  larger  than  the  rest." 
This  proved  to  be  a  new  world,  now  known  as  Uranus. 
The  discovery  led 'in  the  following  year  to  his  appointment 
as  astronomer  to  the  king,  George  III,  with  a  salary  suf- 
ficient to  enable  him  to  devote  his  whole  time  to  astronomy. 

One  of  the  fruits  of  this  increased  leisure  was  the  con- 
struction of  a  telescope  far  more  powerful  than  had  been 
dreamed  of  by  his  predecessors — namely,  a  telescope  four 
feet  in  diameter  and  forty  feet  in  length.  Commenced  in 
1785,  Herschel  dated  its  completion  as  August  28,  1789, 
when  he  discovered  by  its  means  a  sixth  satellite  of  Saturn, 
and,  less*  than  a  month  later,  a  seventh,  even  closer  to  the 
planet  and  smaller  than  the  sixth.  We  may  regard  this 
achievement  as  marking  the  limit  of  progress  in  the  reflect- 
ing telescope,  for,  although  at  least  one  as  large  is  now  in 
use,  and  one  even  half  as  large  again  has  been  constructed, 
it  is  more  than  doubtful  whether  they  were  ever  as  perfect 
as  Herschel's  at  its  best. 

There  has  been  one  improvement,  however,  in  the  re- 
flecting telescope  since  the  time  of  Herschel  which  ought 
not  to  be  left  unnoticed  here — namely,  that  of  replacing 
the  heavy  metal  mirror  by  one  of  glass,  made  even  more 
highly  reflective  than  the  old  mirrors  by  a  thin  coating  of 
silver  deposited  by  chemical  methods  upon  the  polished 
glass.  The  great  advantage  of  this  modern  form  of  reflector 
lies  not  so  much  in  the  greater  lightness  and  rigidity  of 
the  material  as  in  that  the  surface  when  tarnished  can  be 
renewed  by  the  simple  process  of  replacing  the  old  silver 
film  by  a  new  one;  whereas  in  the  metal  reflectors  a  tar- 


350 


HASTINGS 


nished  surface  required  a  repetition  of  the  most  difficult  and 
critical  portion  of  the  whole  process  of  construction.  The 
construction  is  also  so  comparatively  simple  that  an  efficient 
reflector  is  far  less  expensive  than  are  refracting  telescopes 
of  like  power,  so  that  this  may  be  regarded  as  particularly 
the  amateur's  telescope.  On  the  other  hand,  such  tele- 
scopes are,  like  their  predecessors,  extremely  inconstant, 
and  they  require  much  more  careful  attention  to  keep  them 
in  working  order.  It  is  for  these  reasons,  doubtless,  that 
silver-on-glass  reflectors  have  done  so  little  for  the  advance- 
ment of  astronomical  discovery.  In  astronomical  photog- 
raphy, however,  they  promise  to  do  much;  and,  indeed, 
at  the  present  date  by  far  the  best  photographs  we  have  of 
any  nebulae  have  been  made  by  Mr.  Common's  magnificent 
reflector  of  three  feet  in  diameter,  and  by  the  twenty-inch 
reflector  of  Mr.  Roberts. 

We  must  go  back  now  to  a  quarter  of  a  century  before 
Herschel  discovered  the  new  planet,  to  the  very  year,  in- 
deed, when  that  great  astronomer  first  set  foot  on  English 
soil,  in  order  to  trace  the  history  of  another  form  of  tele- 
scope which  has  remained  unrivalled  for  the  last  half  cen- 
tury in  the  more  difficult  fields  of  astronomical  research, 
and  which  to-day  finds  its  most  perfect  development  in  the 
instruments  at  Mount  Hamilton,  at  Pultowa,  at  Vienna, 
and  at  Washington. 

Newton  had  declared  that,  as  a  result  from  his  experi- 
ments, separation  of  white  light  into  its  constituent  colours 
was  an  inevitable  accompaniment  of  deviation  by  refraction, 
and  consequently  the  shortening  of  the  unwieldy  refractors 
was  impracticable.  The  correctness  of  the  experiments  re- 
mained unquestioned  for  nearly  a  century;  but  a  famous 
German  mathematician,  Euler,  did  question  his  conclusion. 
His  argument  was  that  since  the  eye  does  produce  colour- 
less images  of  white  objects  it  might  be  possible  by  the 
proper  selection  of  curves  to  so  combine  lenses  of  glass  and 
of  water  as  to  produce  a  telescope  free  from  the  colour  de- 
fect. Although  Euler's  premise  was  an  error,  since  the  eye 
is  not  free  from  dispersion,  his  efforts  had  the  effect  of  lead- 


THE   HISTORY  OF  THE   TELESCOPE  351 

ing  to  much  more  critical  study  of  the  phenomena  involved. 
In  this  John  Dolland,  an  English  optician,  met  with  bril- 
liant success.  Repeating  an  experiment  of  Newton's  with 
a  prism  of  water  opposed  by  a  prism  of  glass  he  found  that 
deviation  of  light  could  be  produced  without  accompany- 
ing dispersion  into  prismatic  colours.  More  than  this,  he 
found  that  the  two  varieties  of  glass,  then  as  now  common 
in  England — crown  or  common  window  glass,  and  flint 
glass,  which  is  characterized  by  the  presence  of  a  greater  or 
less  quantity  of  lead  oxide — possessed  very  different  powers 
in  respect  to  dispersion;  thus,  of  two  prisms  of  these  two 
varieties  of  glass  which  would  deflect  the  light  by  the  same 
angle,  that  made  of  flint  glass  would  form  a  spectrum  nearly 
twice  as  long  as  the  other;  hence,  if  a  prism  of  crown  glass 
deflecting  a  transmitted  beam  of  light,  say  ten  degrees,  were 
combined  with  one  of  flint  glass  which  would  deflect  the 
beam  of  light  five  degrees  in  the  opposite  direction,  there 
would  remain  a  deflection  of  five  degrees  without  division 
into  colour.  It  also  follows  that  a  positive  lens  of  crown 
combined  with  a  negative  lens  of  flint  of  half  the  power 
would  yield  a  colourless  image.  Such  combinations  of  two 
different  substances  are  called  achromatic  systems.  It  is 
a  singular  fact,  worth  noting  in  passing,  that  more  than 
twenty  years  before  Dolland's  success,  Mr.  Chester  More 
Hall  had  invented  and  made  achromatic  telescopes,  but  this 
remained  unknown  to  the  world  of  science  until  after  Dol- 
land's telescopes  became  famous. 

For  a  long  time  this  ingenious  invention  remained 
fruitless  for  astronomical  discovery  (though  they  were  early 
applied  to  meridian  instruments),  on  account  of  the  im- 
possibility of  securing  sufficiently  large  and  perfect  pieces 
of  glass,  more  particularly  of  flint  glass.  Not  until  after 
the  beginning  of  this  century  was  any  real  advance  in  this 
branch  of  the  arts  exhibited.  Even  then  success  appeared, 
not  in  England  or  France,  where  most  strenuous  efforts  had 
been  made  to  improve  the  quality  of  optical  glass,  but  in 
Switzerland.  There  a  humble  mechanic,  a  watchmaker 
named  Guinaud,  spent  many  years  in  efforts,  long  unfruit- 


352  HASTINGS 

ful,  to  make  large  pieces  of  optical  glass.  What  degree  of 
success  he  attained  there  during  twenty  years  of  experi- 
ment we  do  not  know,  though  from  the  fact  that  during 
that  period  good  achromatic  telescopes  of  more  than  five 
inches  in  diameter  were  unknown,  we  must  conclude  that 
his  success  was  limited.  In  1805  he  joined  the  optical 
establishment  of  Fraunhofer  and  Utzschneiden  in  Munich. 
Here  he  remained  nine  years,  and  with  the  increased  means 
at  his  disposal,  and  the  aid  of  Fraunhofer,  he  perfected  his 
methods  so  far  that  the  production  of  large  disks  of  homo- 
geneous glass  became  only  a  matter  of  time  and  cost — that 
is  to  say,  all  of  the  large  pieces  of  optical  glass  which  have 
since  been  produced,  whether  in  Germany,  France,  or  Eng- 
land, have  been  made  by  direct  heirs  of  the  practical  secrets 
of  this  Swiss  watchmaker. 

Fraunhofer  was  a  genius  of  a  high  order.  Although  he 
died  at  the  early  age  of  thirty-nine,  he  had  not  only  brought 
the  achromatic  telescope  to  a  degree  of  optical  perfection 
which  made  it  a  rival  of  the  most  powerful  of  the  reflector 
type,  and  so  far  improved  its  method  of  mounting  that  his 
system  has  replaced  all  others;  but  he  also  made  some 
capital  discoveries  in  the  domain  of  physical  optics.  His 
great  achievement  was  the  construction  of  an  achromatic 
telescope  9.6  inches  in  diameter,  with  which  the  elder  Struve 
made  at  Dorpat  his  remarkable  series  of  discoveries  and 
measurements  of  double  stars.  The  character  of  Struve's 
work  demonstrates  the  excellence  of  the  telescope,  and 
shows  us  that  it  is  to  be  ranked  as  the  equal  of  all  but  the 
very  best  of  its  predecessors.  Indeed,  it  may  fairly  be  con- 
cluded that  not  more  than  one  or  two  telescopes,  and  those 
made  and  used  by  Herschel,  had  ever  been  of  greater  power, 
while  in  convenience  for  use  the  new  refractor  was  vastly 
superior. 

For  a  long  time  Fraunhofer  and  his  successors,  Merz 
and  Mahler,  from  whom  the  great  telescopes  of  Pultowa 
and  of  the  Harvard  Observatory  were  procured,  remained 
unrivalled  in  this  field  of  optics.  But  they  have  been  fol- 
lowed by  a  number  of  skilful  constructors  whose  products 


THE   HISTORY   OF   THE   TELESCOPE  353 

have,  since  the  middle  of  the  century,  been  scattered  all 
over  the  world.  In  Germany,  Steinheil  and  Schroder;  in 
France,  Canchois,  Martin,  and  the  Henry  brothers;  in  Eng- 
land, Cook  and  Grubb;  and  in  this  country  the  Clarks  and 
Brashear,  have  each  produced  one  or  more  great  telescopes 
which  has  rendered  his  name  familiar  to  all  readers  of 
astronomical  history.  Of  these  the  Clarks,  father  and  son, 
have  beyond  a  doubt  won  the  first  place,  whether  deter- 
mined by  the  character  of  the  discoveries  made  by  means 
of  their  instruments  or  by  the  fact  that  the  two  most  power- 
ful telescopes  in  existence  were  made  by  them — namely, 
the  new  refractor  of  thirty  inches  in  diameter  at  Pultowa, 
and  the  great  refractor  of  three  feet  diameter  of  the  Lick 
Observatory  in  California.  The  most  notable  discoveries 
made  with  their  telescopes  are  the  satellites  of  Mars  and  the 
companion  to  Sirius;  but  besides  these  there  is  a  long  list 
of  double  stars  of  the  most  difficult  character  discovered 
by  the  makers  themselves,  by  Dawes  in  England,  by  Burn- 
ham  in  our  own  country,  and  by  a  number  of  other  ob- 
servers. 

We  ought  not  to  terminate  our  review  of  the  develop- 
ment of  the  telescope  without  a  reference  to  the  parallel 
development  of  the  mounting  of  great  telescopes.  Indeed, 
did  this  not  lead  us  too  far  from  the  immediate  aim  in  view, 
we  might  find  a  great  deal  of  interest  and  be  brought  into 
agreeable  contact  with  some  of  the  cleverest  mechanicians 
and  engineers  of  two  centuries  by  tracing  its  course.  We 
should  meet  with  Huygens,  as  the  inventor  of  the  aerial 
telescope,  and  perhaps  consider  the  claims  of  his  contem- 
porary, Robert  Hook,  as  a  rival  inventor,  for  we  may  be 
sure  that  nothing  which  brings  us  to  a  study  of  that  curious 
and  able  philosopher  would  fail  to  possess  interest.  We 
should  find  Herschel  confronted  with  the  problem  as  to 
how  he  should  use  his  great  forty-foot  telescope,  and  the 
study  of  his  solution  would  guide  us  in  valuing  the  results 
of  the  subsequent  efforts  of  Lassel  and  Rosse.  The  same 
line  of  study  would  bring  us  to  Grubb's  clever  and  inter- 
esting equatorial  mounting  of  that  anachronism,  the  four- 
23 


354 


HASTINGS 


foot  Melbourne  reflector.  But  we  should  find  nothing  of 
very  notable  interest  in  the  mounting  of  refractors,  after  the 
time  of  Huygens  and  Hook,  until  Fraunhofer  invented  a 
type  of  mounting  for  the  famous  Dorpat  equatorial,  which 
still  remains  in  its  essential  features  as  the  type  in  universal 
use.  With  the  increase  in  size  of  the  telescopes  to  be  di- 
rected toward  the  heavens,  however,  the  number  and  com- 
plexity of  the  mechanical  problems  to  be  solved  has  been 
vastly  increased,  so  that  they  have  taxed  the  best  powers 
of  some  of  the  ablest  mechanicians.  The  Repsolds,  of  Ger- 
many, and  Sir  Howard  Grubb,  of  Dublin,  have  specially 
distinguished  themselves  in  this  field  of  activity.  But  it 
seems  to  me  that  none  have  shown  greater  fertility  of  re- 
sources, greater  skill  in  the  solution  of  every  problem  af- 
fecting the  comfort  and  efficacy  of  the  observer,  and  greater 
taste,  combined  with  accurate  workmanship,  than  have  the 
celebrated  firm  which  has  mounted  the  telescope  at  Mount 
Hamilton  and  that  at  Carleton  College. 

We  come  now  to  a  consideration  of  the  present  state  of 
the  art  of  lens-making.  We  ask  why  such  a  very  large  pro- 
portion of  the  telescopes  in  existence  are  bad;  why  there 
was  a  time,  brief  it  is  true,  during  which  the  glass-maker 
was  certainly  in  advance  of  the  demands  of  telescope- 
makers;  and  why,  finally,  the  first  of  the  great  modern  ob- 
jectives was  in  the  hands  of  the  most  skilful  optician  in 
Great  Britain  for  seven  years,  and  even  then  this  maker 
asserted  that  it  was  incomplete. 

These  questions  can  not  be  answered  in  a  word,  but  we 
can,  at  least,  gain  much  in  perspicuity  by  recognising  that 
the  reasons  are  of  two  distinct  kinds — namely,  purely  tech- 
nical and  theoretical;  and  by  regarding  them  briefly  in  suc- 
cession. 

The  art  of  lens-making  can  be  certainly  traced  back  to 
the  thirteenth  century,  though  the  methods  at  a  much  later 
day  than  that  were  so  rude  that,  as  we  have  seen,  Galileo 
had  the  utmost  difficulty  in  making  a  lens  good  enough 
to  bear  a  magnifying  power  of  thirty  times.  At  the  present 
day  there  is  little  difficulty  in  selecting  a  spectacle  glass 


THE   HISTORY   OF   THE   TELESCOPE  355 

which  would  rival  that  most  famous  of  all  telescopes.  Not 
until  after  another  generation  of  effort  was  there  such 
notable  improvement  in  the  technique  of  lens-making  that 
further  astronomical  discovery  was  possible.  The  reasons 
for  this  slow  progress  are  to  be  found  in  the  extremely 
critical  requirements  for  a  good  lens.  A  departure  by  a 
fraction  of  r^Vinr  Part  °f  an  mcn  horn  a  correct  geo- 
metrical surface  will  greatly  impair  the  performance  of  an 
objective.  But  even  at  this  day  the  limit  of  accurate  meas- 
urement may  be  set  at  about  1 0  0*0  0  0  of  an  inch,  while 
it  is  quite  probable  that  ten  times  that  value  was  vanishingly 
small  to  the  artisans  of  a  century  or  more  ago.  It  was 
necessary,  therefore,  to  devise  a  method  of  polishing — for 
it  is  a  comparatively  simple  matter  to  grind  a  surface  accu- 
rately— which  should  keep  the  surface  true  within  a  limit 
far  transcending  the  range  of  measurements.  Huygens  is 
the  first  who  seems  to  have  done  this,  by  polishing  upon  a 
paste  which  was  formed  to  the  glass  and  then  dried,  and  by 
using  only  the  central  portion  of  a  large  lens.  In  Italy 
Campani  developed  a  system  which  he  most  jealously 
guarded  as  a  secret  until  his  death,  consisting  of  polishing 
with  a  dry  powder  on  paper  cemented  to  the  grinding  tools. 
This  method  still  survives  in  Paris  to  the  exclusion  of 
almost  all  others,  and  it  is  probably  the  best  for  work  which 
does  not  demand  the  highest  scientific  precision. 

Newton,  however,  was  the  first  to  introduce  a  method 
which  has  since  been  developed  to  a  state  of  surprising 
delicacy.  Casting  about  for  a  means  which  should  be  suf- 
ficiently "  tender,"  to  use  his  own  expression,  for  polishing 
the  soft  speculum  metal,  he  fixed  upon  pitch,  shaped  to 
the  mirror  while  warm,  as  a  bed  to  hold  the  polishing  pow- 
der. But  the  enormous  value  of  this  substance  lies  not  so 
much  in  the  comparative  immunity  which  it  gives  from 
scratching,  but  in  the  fact  that  under  slowly  changing  forces 
it  is  a  liquid,  but  under  those  of  short  duration  it  behaves 
like  a  hard  and  brittle  solid.  Thus  it  is  possible  to  slowly 
alter  the  shape  of  a  lens  while  polishing,  in  any  desired 
direction.  It  was  only  after  the  practical  recognition  of 


'• 


356  HASTINGS 

this  fact  that  really  excellent  lenses  were  much  more  than 
a  question  of  good  fortune.  The  perfecting  of  this  method 
belongs  without  doubt  to  the  English  of  the  last  century 
and  the  early  part  of  this.  In  the  "  Philosophical  Transac- 
tions "  we  find  many  long  papers  relating  to  this  art,  con- 
tributed by  skilful  and  successful  amateurs.  We  may  there- 
fore regard  the  technique  of  the  art  of  lens-making  as  prac- 
tically complete  at  the  middle  of  this  century  and  as  com- 
mon property,  so  that  success  no  longer  depends  upon  the 
holding  of  some  special  or  secret  method. 

We  are  now  (after  this,  I  fear,  somewhat  dry  discussion 
of  a  necessary  point)  in  a  condition  to  explain  the  differ- 
ences between  the  processes  pursued  by  most  telescope- 
makers  and  that  of  the  maker  of  the  Carleton  College 
telescope. 

This  is  the  ordinary  method:  After  securing  perfect 
pieces  of  glass,  crown  and  flint,  as  like  as  possible  to  those 
generally  used,  and  having  fixed  upon  the  general  shape 
of  the  lenses,  a  guess  is  made  as  to  the  proper  radii  of  the 
four  surfaces  to  determine  the  desired  focal  length  and  cor- 
rections both  for  colour  and  spherical  aberration.  The  suc- 
cess of  this  guess  has  much  to  do  with  the  necessary  outlay 
of  labour,  and  therefore  past  experience  is  of  great  value 
as  a  guide.  After  working  the  four  surfaces  to  the  dimen- 
sions provisionally  adopted  so  far  as  to  admit  of  fairly  good 
seeing  through  the  objective,  an  examination  of  the  errors 
is  made.  Should  the  errors  of  colour  be  so  small  that  their 
final  correction  will  not  make  the  telescope  more  than 
from  three  to  ten  per  cent  greater  or  less  than  the  desired 
focal  length,  the  crown  lens  will  probably  be  completed  in 
accordance  with  the  provisional  figures.  Then  the  flint 
lens  will  be  modified  in  such  a  direction  as  will  tend  to 
correct  the  observed  errors  of  colour  and  figure,  until,  by  a 
purely  tentative  process,  the  colour  error  is  practically  neg- 
ligible and  the  error  of  figure  is  small.  Then  follows  a 
process  when  the  qualities  of  skill,  conscientiousness,  and 
perseverance  have  full  scope.  This  process,  first  introduced, 
or  at  least  made  public,  by  Foucault,  is  known  as  local  cor- 


THE   HISTORY  OF  THE  TELESCOPE  357 

recting.  It  consists  in  slowly  polishing  away  portions 
of  the  lens  surfaces  so  that  errors  in  the  focal  image  be- 
come so  small,  not  that  they  can  not  be  detected,  but 
that  one  can  not  determine  whether  they  are  on  the  one 
side  of  truth  or  the  other.  Local  correcting  has  always 
seemed  to  me  to  be  eminently  unscientific  and  unneces- 
sary. It  is  a  process  of  making  errors  which  ought  not 
to  exist. 

Mr.  Brashear's  method  is  essentially  different  from  this. 
Before  the  glasses  are  touched  every  dimension  and  con- 
stant of  the  finished  objective  is  known  with  great  accuracy. 
His  whole  aim  is  to  make  the  surfaces  geometrically  per- 
fect; and  by  ingenious  polishing  machinery,  which  em- 
bodies twelve  years  of  his  thought  and  experience,  he  is 
enabled  to  do  this  with  truly  astonishing  exactness.  All 
the  surfaces  which  admit  of  investigation — usually  three  in 
his  ordinary  construction — are  made  rigidly  true  without 
regard  to  the  character  of  the  focal  image.  This  leaves  only 
one  surface  which  is  known  to  be  very  nearly  a  sphere,  but 
probably  deviating  slightly  within  in  the  direction  of  a 
prolate  or  oblate  spheroid.  A  glance  at  the  character  of 
the  focal  image  will  determine  this  point.  Then  the  polish- 
ing machine  is  adapted  to  bring  about  a  change  in  the 
proper  direction,  and  after  action  during  a  measured  in- 
terval of  time,  the  image  is  again  examined,  and  from  the 
observed  change  in  character  the  necessary  time  for  com- 
plete correction  by  the  same  or  contrary  action  may  be  de- 
duced. It  will  be  observed  that  by  this  means  it  is  quite 
possible  to  correct  errors  which  are  much  too  small  to 
betray  their  nature,  since  a  step  in  the  wrong  direction 
carries  with  it  no  consequences  of  the  slightest  moment, 
since  any  step  may  be  retraced. 

When  we  learn  that  Mr.  Brashear's  telescope  objec- 
tives have  always  had  a  focal  length  differing  only  from  -^ 
to  y^j-  of  one  per  cent  of  the  value  prescribed,  we  have  a 
suggestion  of  the  success  of  his  efforts.  But  adding  to  that 
the  fact  that  he  is  absolutely  untrammelled  by  purely  me- 
chanical considerations,  either  as  to  the  shape  of  his  lenses 


358  HASTINGS 

or  the  character  of  his  materials,  leaving  these  questions 
to  be  decided  alone  by  the  requirements  of  the  astronomer, 
it  seems  to  me  that  we  may  fairly  accord  to  him  the  merit 
of  the  most  important  improvements  introduced  into  his 
art  for  a  very  long  period. 

I  shall  not  venture  to  demand  much  of  your  time  in 
considering  the  purely  theoretical  difficulties  in  telescope 
construction,  not  merely  because  the  subject  has  already 
taxed  our  patience,  but  because  it  would  be  of  almost  too 
technical  a  character  did  we  allow  ourselves  to  regard  any- 
thing but  the  most  general  features. 

The  obvious  requirements  are  that  in  a  good  objective 
the  light  coming  from  a  point  in  the  object  should  be  con- 
centrated at  a  point  in  the  image;  but  this,  combined  with 
a  prescribed  focal  length,  may  be  reduced  to  three  con- 
ditions: First,  a  fixed  focal  length;  second,  freedom  from 
colour  error;  third,  freedom  from  spherical  aberration  for 
a  particular  colour  or  wave-length  of  light.  Now  let  us 
catalogue  what  provisions  we  have  for  satisfying  these 
conditions.  They  are,  four  surfaces,  which  must  be  spheri- 
cal but  may  have  any  radii  we  please,  the  two  thicknesses 
of  the  two  lenses,  and  the  distance  which  separates  the 
lenses — that  is,  seven  elements  which  may  be  varied  to  suit 
our  requirements.  As  a  matter  of  fact,  however,  on  ac- 
count of  the  cost  of  the  material  and  the  fact  that  glass  is 
perfectly  transparent,  for  powerful  telescopes  we  must  make 
the  lenses  as  thin  as  possible;  and  we  shall  find  also  that 
separating  the  lenses  introduces  errors  away  from  the  axis 
which  are,  to  say  the  least,  undesirable.  We  have  left, 
therefore,  only  the  four  radii  as  arbitrary  constants.  These, 
however,  are  more  than  enough  to  meet  the  three  require- 
ments. To  make  the  problem  determinate  we  must  add 
another  condition.  The  suggestion  of  this  fourth  condi- 
tion and  carrying  the  problem  to  its  solution  is  the  work 
of  the  great  mathematicians  who  have  directed  their 
thought  to  it.  Clairault  proposed  to  make  the  fourth  con- 
dition that  the  two  adjacent  surfaces  should  fit  together 
and  the  lenses  be  cemented.  This  condition  would  be, 


THE   HISTORY  OF   THE   TELESCOPE  359 

doubtless,  of  great  value  were  it  possible  to  cement  large 
lenses  without  changing  their  shapes  to  a  degree  which 
would  quite  spoil  their  performance.  Sir  John  Herschel 
published  a  very  important  paper  in  1821,  in  which  he  made 
the  fourth  condition  that  the  spherical  aberration  should 
vanish,  not  only  for  objects  at  a  very  great  distance,  but 
also  for  those  at  a  moderate  distance.  In  this  paper  he 
computed  a  table,  afterward  greatly  extended  by  Prof. 
Baden-Powell,  for  the  avowed  purpose  of  aiding  the  prac- 
tical optician.  It  was  this  feature  undoubtedly  which 
brought  his  construction,  not  at  all  a  good  one,  as  we  shall 
see,  into  more  general  use  than  any  other  for  some  time. 
But,  as  all  Herschel's  tables  were  derived  from  calculations 
which  wholly  disregarded  the  thickness  of  the  lenses,  I  am 
quite  unable  to  see  how  they  could  have  been  of  any 
material  aid,  and  am  inclined  to  suspect  that  the  discredit 
with  which  opticians  have  received  the  dicta  of  mathema- 
ticians concerning  their  instruments  may  have  been  due  in 
part  to  this  very  fact.  It  is  a  singular  fact,  for  which  I  have 
in  vain  sought  the  explanation,  that  Fraunhofer's  objec- 
tives are  of  just  such  a  form  as  to  comply  with  the  Her- 
schelian  solution,  although  they  must  have  been  made  quite 
independently. 

Gauss  made  the  fourth  condition  that  another  colour  or 
wave-length  of  light  should  be  also  free  from  spherical  aber- 
ration. This  seems  to  have  been  a  tour  de  force  as  a  mathe- 
matician, not  as  a  sober  suggestion  of  an  improvement  in 
construction,  for  in  a  point  of  fact  the  construction  is  very 
bad.  It  was  generally  believed  that  this  condition  could 
not  be  fulfilled;  therefore  Gauss,  who  was  particularly  fond 
of  doing  what  all  the  rest  of  the  world  believed  impossible, 
straightway  did  it.  There  has  been  only  one  effort  to  carry 
out  this  suggestion  of  Gauss,  and  that  forty  years  later  by 
Steinheil,  but  it  proved  a  disappointment.  A  much  larger 
objective  made  by  Clark  a  few  years  ago,  of  the  general 
form  of  Gauss's  objective,  probably  does  not  meet  the 
Gaussian  condition;  at  least,  this  condition  is  extremely 
critical,  and  I  believe  it  is  not  asserted  that  the  objective 


360  HASTINGS 

was  ever  thoroughly  investigated.  It  has  been  the  father 
of  no  others. 

It  is  hardly  surprising,  since  none  of  these  forms  have 
any  real  merit,  that  the  practical  optician  has,  following  the 
line  of  least  resistance,  adopted  a  form  which  costs  him  less 
labour  than  those  heretofore  mentioned,  and  is  quite  as 
good.  By  making  the  curve  equi-convex  the  trouble  and 
expense  of  making  one  pair  of  tools  are  saved,  although 
this  would  hardly  appear  a  satisfactory  reason  for  choice  of 
a  particular  form  to  the  astronomer,  who  simply  demands 
the  best  possible  instrument  of  research. 

The  reason  for  so  much  futile  work  on  the  theory  of  the 
telescope  objective  is  not  far  to  seek.  It  had  always  been 
tacitly  assumed  that  the  condition  of  colour  correction,  one 
of  those  which  serves  to  determine  the  values  of  the  arbi- 
trary constants,  was  readily  determinable — in  fact,  one  of 
the  donne  of  the  problem,  whereas  it  is  just  this  datum 
which  has  offered  peculiar  difficulties.  Fraunhofer  brought 
all  the  resources  at  the  command  of  his  genius  to  bear  upon 
this  point,  and  frankly  failed,  although  in  the  effort  he 
made  a  splendid  discovery,  which  has  assured  a  permanence 
to  his  fame  no  less  than  that  of  the  history  of  science  itself 
— the  discovery  of  the  dark,  or  Fraunhofer,  lines  in  solar 
and  stellar  spectra.  Gauss  proposed  the  condition  that  the 
best  objective  is  that  which  produces  the  most  perfect  con- 
centration of  light  about  the  place  of  the  geometrical  image 
of  a  point,  just  as  the  best  rifle  practice  is  that  which  pro- 
duces the  maximum  concentration  of  hits  about  the  centre 
of  the  target.  That  this  is  a  false  guide  appears  at  once 
from  the  consideration  that  if  we  take  even  as  much  as  ten 
per  cent  of  the  light  from  an  object,  and  diverted  from  the 
image  so  far  that  it  can  not  be  found,  the  telescope  may  still 
be  practically  perfect;  all  of  Herschel's  did  much  worse 
than  this.  But  if  you  take  that  same  ten  per  cent  and  con- 
centrate it  very  close  about  the  image,  the  telescope  will 
be  absolutely  worthless. 

The  true  difficulty  with  most  of  the  theories  is  this: 
There  is  no  recognition  of  the  relative  weight  or  impor- 


THE   HISTORY  OF   THE  TELESCOPE  361 

tance  of  unavoidable  errors.  The  optician  is  confronted  at 
the  very  outset  by  the  fact  that  absolute  elimination  of 
colour  error  is  impossible,  for  certain  physical  reasons  which 
we  have  not  time  for  considering  further.  He  can  reduce 
the  colour  error  of  the  old  single-lens  type  of  telescopes 
hundreds  of  times,  and  hence  the  length  of  the  telescope 
tens  of  times.  It  is  this  fact  which  prevents  the  still  further 
shortening  of  telescopes,  which  keeps  the  ratio  of  length  to 
diameter  not  less  than  fifteen  to  one  in  large  telescopes. 
This  restriction  being  recognised,  let  us  revise  our  limiting 
conditions.  They  now  become,  first,  fixed  focal  length; 
second,  best  colour  correction;  third,  freedom  from  spheri- 
cal aberration  for  a  particular  wave-length  of  light.  We 
therefore  have  still  one  arbitrary  constant  undetermined. 
How  shall  we  fix  its  value,  and  thus  solve  the  problem 
completely?  Surely  there  is  only  one  rational  guide.  Con- 
sider the  residual  errors  and  make  the  fourth  condition  such 
as  to  reduce  these  errors  as  far  as  possible.  Now  the  only 
remaining  errors  are  secondary  colour  error  and  spherical 
aberration  for  colours  other  than  that  for  which  it  is  elimi- 
nated, or,  more  scientifically,  chromatic  difference  of  spheri- 
cal aberration.  Which  of  these  is  the  gravest  defect?  Our 
answer  must  depend  upon  the  use  to  which  the  objective  is 
to  be  put.  If  it  is  a  high-power  microscope  objective,  it 
is  certainly  the  second.  If  it  is  an  objective  to  be  used  for 
photographing  at  considerable  angular  distances  from  the 
axis,  our  question  loses  its  physical  significance,  since  we 
have  excluded  the  consideration  of  eccentric  refraction. 
But  if  the  objective  is  to  be  for  a  visual  telescope,  there  is 
no  question  that  the  defect  of  secondary  colour  error  is 
incomparably  the  most  serious.  Our  fourth  and  determin- 
ing condition  must,  therefore,  be  better  colour  correction. 
These  are  therefore  the  considerations  which  have 
served  as  guides  in  the  construction  of  the  Carleton  College 
objective.  First,  the  selection  of  the  materials  which,  in 
the  present  condition  of  the  art  of  optical  glass-making, 
possess  in  the  highest  degree  the  desired  physical  prop- 
erties; second,  a  general  discussion  of  every  possible  com- 


362  HASTINGS 

bination  of  these  two  pieces  of  glass  and  a  selection  of  the 
forms  which  yield  the  best  attainable  results.  This  consci- 
entious strife  after  scientific  perfection,  the  unexcelled  skill 
with  which  the  results  of  analysis  have  been  interpreted 
into  the  reality  of  substance,  the  gratifying  identity  of  pre- 
dicted and  realized  values  of  physical  characteristics — all 
of  these  have  led  some  of  those  who  have  watched  the 
growth  of  this  new  instrument  of  research  with  the  most 
solicitous  attention  to  the  belief  that  although  not  the  niost 
powerful  in  existence,  it  may  well  be  the  most  perfect  great 
telescope  yet  made.  Let  us  therefore  congratulate  the 
possessors  of  this  noble  instrument,  wish  them  Godspeed 
in  their  search  after  knowledge,  while  we  remind  them  that 
although  no  astronomer  can  ever  make  another  discovery 
which  will  rival  that  made  by  the  insignificant  tube  first 
directed  toward  the  heavens  by  the  Paduan  philosopher,  yet 
no  mind  can  weigh  the  importance  of  any  truth,  however 
trivial  in  appearance,  which  may  be  added  to  that  store 
which  we  call  "  science." 


RESULTS 

OF  SPECTRUM  ANALYSIS 
APPLIED  TO  HEAVENLY  BODIES 

CELESTIAL  SPECTROSCOPY 
THE  NEW  ASTRONOMY 

BY 

WILLIAM   HUGGINS 


THE  RESULTS  OF  SPECTRUM  ANALYSIS 
APPLIED  TO  THE  HEAVENLY  BODIES1 

AJ  important  invention  or  discovery  seldom,  if  ever, 
remains  sterile  and  alone.  It  gives  birth  to  other 
discoveries.  The  telescope  and  the  microscope  have 
led  to  remarkable  discoveries  in  astronomy  and  in  minute 
anatomy  and  physiology,  which  would  not  have  been  pos- 
sible without  those  instruments.  The  observation  that  a 
magnetic  body,  free  to  move,  arranges  itself  nearly  north 
and  south,  has  not  only  contributed  immensely  to  the  ex- 
tension of  commerce  and  of  geographical  discovery,  but 
also  has  founded  the  important  science  of  terrestrial  mag- 
netism. 

This  evening  I  have  to  bring  before  you  some  additions 
to  our  knowledge  in  the  department  of  astronomy,  which 
have  followed  from  a  comparatively  recent  discovery.  The 
researches  of  Kirchhoff  have  placed  in  the  hands  of  the 
astronomer  a  method  of  analysis  which  is  specially  suitable 
for  the  examination  of  the  heavenly  bodies.  So  unexpected 
and  important  are  the  results  of  the  application  of  spectrum 
analysis  to  the  objects  in  the  heavens  that  this  method  of 
observation  may  be  said  to  have  created  a  new  and  distinct 
branch  of  astronomical  science. 

Physical  astronomy,  the  imperishable  and  ever-growing 
monument  to  the  memory  of  Newton,  may  be  described  as 
the  extension  of  terrestrial  dynamics  to  the  heavens.  It 
seeks  to  explain  the  movements  of  the  celestial  bodies  on 
the  supposition  of  the  universality  of  an  attractive  force 
similar  to  that  which  exists  upon  the  earth. 

1  Delivered  before  the  British  Association  for  the  Advancement  of 
Science,  August  23,  1866. 

365 


366  MUGGINS 

The  new  branch  of  astronomical  science  which  spectrum 
analysis  may  be  said  to  have  founded  has  for  its  object  to 
extend  the  laws  of  terrestrial  physics  to  the  other  phe- 
nomena of  the  heavenly  bodies,  and  it  rests  upon  the  now 
established  fact  that  matter  of  a  nature  common  to  that  of 
the  earth,  and  subject  to  laws  similar  to  those  which  prevail 
upon  the  earth,  exists  throughout  the  stellar  universe. 

The  peculiar  importance  of  KirchhofFs  discovery  to 
astronomy  becomes  obvious  if  we  consider  the  position  in 
which  we  stand  to  the  heavenly  bodies.  Gravitation  and 
the  laws  of  our  being  do  not  permit  us  to  leave  the  earth; 
it  is,  therefore,  by  means  of  light  alone  that  we  can  obtain 
any  knowledge  of  the  grand  array  of  worlds  which  sur- 
round us  in  cosmical  space.  The  starlit  heavens  is  the 
only  chart  of  the  universe  we  have,  and  in  it  each  twinkling 
point  is  the  sign  of  an  immensely  vast  though  distant  re- 
gion of  activity. 

Hitherto  the  light  from  the  heavenly  bodies,  even  when 
collected  by  the  largest  telescope,  has  conveyed  to  us  but 
very  meagre  information,  and  in  some  cases  only  of  their 
form,  their  size,  and  their  colour.  The  discovery  of  Kirch- 
hoff  enables  us  to  interpret  symbols  and  indications  hidden 
within  the  light  itself,  which  furnish  trustworthy  informa- 
tion of  the  chemical,  and  also  to  some  extent  of  the  physi- 
cal, condition  of  the  excessively  remote  bodies  from  which 
the  light  has  emanated. 

We  are  indebted  to  Newton  for  the  knowledge  that  the 
beautiful  tints  of  the  rainbow  are  the  common  and  neces- 
sary ingredients  of  ordinary  light.  He  found  that  when 
white  light  is  made  to  pass  through  a  prism  of  glass  it  is 
decomposed  into  the  beautiful  colours  which  are  seen  in 
the  rainbow.  These  colours  when  in  this  way  separated 
from  each  other  form  the  spectrum  of  the  light.  Let  this 
white  plate  represent  the  transverse  section  of  a  beam  of 
white  light  travelling  toward  you.  Let  now  a  prism  be 
interposed  in  its  path.  The  beam  of  white  light  is  not 
turned  aside  as  a  whole,  but  the  coloured  lights  composing 
it  are  deflected  differently,  each  in  proportion  to  the  rapidity 


SPECTRUM   ANALYSIS  367 

of  its  vibrations.  An  obvious  consequence  will  be  that,  on 
emerging  from  the  prism,  the  coloured  lights  which  formed 
the  white  light  will  separate  from  each  other,  and  in  place 
of  the  white  light  which  entered  the  prism  we  shall  have  its 
spectrum — that  is,  the  coloured  lights  which  composed  it — 
in  a  state  of  separation  from  each  other.  Wollaston  and 
Fraunhofer  discovered  that  when  the  light  of  the  sun  is 
decomposed  by  a  prism,  the  rainbow  colours  which  form 
its  spectrum  are  not  continuous,  but  are  interrupted  by  a 
large  number  of  dark  lines.  These  lines  of  darkness  are  the 
symbols  which  indicate  the  chemical  constitution  of  the 
sun.  It  was  not  until  recently,  in  the  year  1859,  that  Kirch- 
hoff  taught  us  the  true  nature  of  these  lines.  He  himself 
immediately  applied  his  method  of  interpretation  to  the 
dark  lines  of  the  solar  spectrum,  and  was  rewarded  by  the 
discovery  that  several  of  the  chemical  elements  which  exist 
upon  the  earth  are  present  in  the  solar  atmosphere. 

It  is  my  intention  to  bring  before  you  this  evening  the 
results  of  the  extension  of  this  method  of  analysis  to  the 
heavenly  bodies  other  than  the  sun.  These  researches  have 
been  carried  on  in  my  observatory  during  the  last  four  years. 
In  respect  of  a  large  part  of  these  investigations — viz.,  those 
of  the  moon,  the  planets,  and  fixed  stars — I  have  had  the 
great  pleasure  of  working  conjointly  with  the  very  distin- 
guished chemist  and  philosopher,  Dr.  William  A.  Miller. 
Half  a  century  ago  Fraunhofer  recognised  several  of  the 
solar  lines  in  the  light  of  the  moon,  Venus,  and  Mars,  and 
also  in  the  spectra  of  several  stars.  Recently  Donati,  Jans- 
sen,  Secchi,  Rutherfurd,  and  the  Astronomer  Royal  have 
observed  lines  in  the  spectra  of  some  stars.  Before  I  de- 
scribe the  results  of  our  observations  I  will  state,  in  a  few 
words,  the  principles  of  spectrum  analysis  upon  which  our 
interpretation  of  the  phenomena  we  have  observed  has  been 
based,  and  also  the  method  of  observing  which  we  have 
employed. 

When  light  which  has  emanated  from  different  sources 
is  decomposed  by  a  prism,  the  spectra  which  are  obtained 
may  differ  in  several  important  respects  from  each  other. 


368  HUGGINS 

All  the  spectra  which  may  present  themselves  can  be  con- 
veniently arranged  in  three  general  groups.  A  spectrum 
illustrating  each  of  these  three  orders  is  placed  upon  the 
diagram : 

1.  The  special  character  which  distinguishes  spectra  of 
the  first  order  consists  in  that  the  continuity  of  the  coloured 
band  is  unbroken  either  by  dark  or  bright  lines.    By  means 
of  the  electric  lamp,  Mr.  Ladd  will  throw  a  spectrum  of  this 
order  upon  the  screen.     We  learn  from  such  a  spectrum 
that  the  light  has  been  emitted  by  an  opaque  body,  and 
almost  certainly  by  matter  in  the  solid  or  liquid  state.     A 
spectrum  of  this  order  gives  to  us  no  knowledge  of  the 
chemical  nature  of  the  incandescent  body  from  which  light 
comes.     In  the  present  case  the  light  is  emitted  by  the 
white-hot  carbon  points  of  the  electric  lamp.    A  spectrum 
in  all  respects  similar  would  be  formed  by  the  light  from 
incandescent  iron,  or  lime,  or  magnesia. 

2.  Spectra  of  the  second  order  are  very  different.    These 
consist  of  coloured  lines  of  light  separated  from  each  other. 
From  such  a  spectrum  we  may  learn  much.    It  informs  us 
that  the  luminous  matter  from  which  the  light  has  come  is 
in  the  state  of  gas.     It  is  only  when  a  luminous  body  is 
free  from  the  molecular  trammels  of  solidity  and  liquidity 
that  it  can  exhibit  its  own  peculiar  power  of  radiating  some 
coloured  rays  alone.    Hence  substances,  when  in  a  state  of 
gas,  may  be  distinguished  from  each  other  by  their  spectra. 
Each  element,  and  every  compound  body  that  can  become 
luminous  in  the  gaseous  state  without  suffering  decomposi- 
tion, is  distinguished  by  a  group  of  lines  peculiar  to  itself. 
These  green  lines  are  produced  by  silver  in  a  state  of  gas, 
and  only  by  silver  gas.    It  is  obvious  that  if  the  groups  of 
lines  characterizing  the  different  terrestrial  substances  be 
known,  a  comparison  of  these  as  standard  spectra  with  the 
spectrum  of  light  from  an  unknown   source  will   show 
whether  any  of  these  terrestrial  substances  exist  in  the 
source  of  the  light. 

3.  The  third  order  consists  of  the  spectra  of  incan- 
descent solid  or  liquid  bodies,  in  which  the  continuity  of 


SPECTRUM   ANALYSIS  369 

the  coloured  light  is  broken  by  dark  lines.  These  dark 
spaces  are  not  produced  by  the  source  of  the  light.  They 
tell  us  of  vapours  through  which  the  light  has  passed  on  its 
way,  and  which  have  robbed  the  light,  by  absorption,  of  cer- 
tain definite  colours  or  rates  of  vibration;  such  spectra  are 
formed  by  the  light  of  the  sun  and  stars. 

Kirchhoff  has  shown  that  if  vapours  of  terrestrial  sub- 
stances come  between  the  eye  and  an  incandescent  body 
they  cause  groups  of  dark  lines,  and,  further,  that  the  group 
of  dark  lines  produced  by  each  vapour  is  identical  in  the 
number  of  the  lines  and  in  their  position  in  the  spectrum 
with  the  group  of  bright  lines  of  which  its  light  consists 
when  the  vapour  is  luminous. 

Mr.  Ladd  will  throw  upon  the  screen  the  spectrum  of 
incandescent  carbon  points  which  contain  sodium.  Ob- 
serve in  addition  to  the  continuous  spectrum  of  the  incan- 
descent carbon  a  bright-yellow  band,  which  indicates  the 
presence  of  sodium.  Now  a  piece  of  metallic  sodium  will 
be  introduced  into  the  lamp.  The  sodium  will  be  va- 
pourized  by  the  heat,  and  will  fill  the  lamp  with  its  vapour. 
This  vapour  absorbs,  quenches  the  light  that  it  emits 
when  luminous.  There  will  thus  be  produced  a  black 
line  exactly  in  the  place  where  the  bright-yellow  line  was 
seen. 

It  is  evident  that  Kirchhoff  by  this  discovery  has  fur- 
nished us  with  the  means  of  interpreting  the  dark  lines 
of  the  solar  spectrum.  For  this  purpose  it  is  necessary  to 
compare  the  bright  lines  in  the  spectra  of  the  light  of  ter- 
restrial substances,  when  in  the  state  of  gas,  with  the  dark 
lines  in  the  solar  spectrum.  When  a  group  of  bright  lines 
coincides  with  a  similar  group  of  dark  lines,  then  we  know 
that  the  terrestrial  substance  producing  the  bright  lines  is 
present  in  the  atmosphere  of  the  sun;  for  it  is  this  sub- 
stance, and  this  substance  alone,  which,  by  its  own  pecul- 
iar power  of  absorption,  can  produce  that  particular  group 
of  dark  lines.  In  this  way  Kirchhoff  discovered  the  pres- 
ence of  several  terrestrial  elements  in  the  solar  atmos- 
phere. 

24 


370  HUGGINS 

METHODS  OF  OBSERVATION 

I  now  pass  to  the  special  methods  of  observation  by 
which,  in  our  investigations,  we  have  applied  these  prin- 
ciples of  spectrum  analysis  to  the  light  of  the  heavenly 
bodies.  I  may  here  state  that  several  circumstances  unite 
to  make  these  observations  very  difficult  and  very  irksome. 
In  our  climate,  on  few  only  even  of  those  nights  in  which 
the  stars  shine  brilliantly  to  the  naked  eye,  is  the  air  suf- 
ficiently steady  for  these  extremely  delicate  observations. 
Further,  the  light  of  the  star  is  feeble.  This  difficulty  has 
been  met,  in  some  measure,  by  the  employment  of  a  large 
telescope.  The  light  of  a  star  falling  upon  the  surface  of 
an  object-glass  of  eight  inches  aperture  is  gathered  up  and 
concentrated  at  the  focus  into  a  minute  and  brilliant  point 
of  light. 

Another  inconvenience  arises  from  the  apparent  move- 
ment of  the  stars,  caused  by  the  rotation  of  the  earth,  which 
carries  the  astronomer  and  his  instruments  with  it.  This 
movement  was  counteracted  by  a  movement  given  by 
clockwork  to  the  telescope  in  the  opposite  direction.  In 
practice,  however,  it  is  not  easy  to  retain  the  image  of  a 
star  for  any  length  of  time  exactly  within  the  jaws  of  a 
slit  only  the  ^  of  an  inch  apart.  By  patient  perseverance 
these  difficulties  have  been  overcome,  and  satisfactory  re- 
sults obtained.  We  considered  that  the  trustworthiness 
of  our  results  must  rest  chiefly  upon  direct  and  simultane- 
ous comparison  of  terrestrial  spectra  with  those  of  celestial 
objects.  For  this  purpose  we  contrived  the  following  ap- 
paratus. 

By  an  outer  tube  the  instrument  is  attached  to  the 
eye  end  of  the  telescope,  and  is  carried  round  with  it  by 
the  clock  motion.  Within  this  outer  tube  a  second  tube 
slides,  carrying  a  cylindrical  lens.  This  lens  is  for  the  pur- 
pose of  elongating  the  round  pointlike  image  of  the  star 
into  a  short  line  of  light,  which  is  made  to  fall  exactly  within 
the  jaws  of  a  nearly  closed  slit.  Behind  the  slit  an  achro- 
matic lens  (and  at  the  distance  of  its  own  focal  length) 


SPECTRUM   ANALYSIS  371 

causes  the  pencils  to  emerge  parallel.  They  then  pass  into 
two  prisms  of  dense  flint  glass.  The  spectrum  which  results 
from  the  decomposition  of  the  light  by  the  prisms  is  viewed 
through  a  small  achromatic  telescope.  This  telescope  is 
provided  with  a  micrometer  screw,  by  which  the  lines  of 
the  spectrum  may  be  measured. 

The  light  of  the  terrestrial  substances  which  are  to  be 
compared  with  the  stellar  spectra  is  admitted  into  the  in- 
strument in  the  following  manner: 

Over  one  half  of  the  slit  is  fixed  a  small  prism,  which 
receives  the  light  reflected  into  it  by  the  movable  mirror 
placed  above  the  tube.  The  mirror  faces  a  clamp  of  ebonite, 
provided  with  forceps  to  contain  fragments  of  the  metals 
employed.  These  metals  are  rendered  luminous  in  the  state 
of  gas  by  the  intense  heat  of  the  sparks  from  a  powerful 
induction  coil.  The  light  from  the  spark  reflected  into  the 
instrument  by  means  of  the  mirror  and  the  little  prism 
passes  on  to  the  prisms  in  company  with  that  from  the  star. 
In  the  small  telescope  the  two  spectra  are  viewed  in  juxta- 
position, so  that  the  coincidence  and  relative  positions  of 
the  bright  lines  in  the  spectrum  of  the  spark  with  dark  lines 
in  the  spectrum  of  the  star  can  be  accurately  determined. 

MOON   AND    PLANETS 

I  now  pass  to  the  results  of  our  observations. 

I  refer  in  a  few  words  only  to  the  moon  and  planets. 
These  objects,  unlike  the  stars  and  nebulae,  are  not  original 
sources  of  light.  Since  they  shine  by  reflecting  the  sun's 
light,  their  spectra  resemble  the  solar  spectrum,  and  the 
only  indications  in  their  spectra  which  may  become  sources 
of  knowledge  to  us  are  confined  to  any  modifications  which 
the  solar  light  may  have  suffered,  either  in  the  atmosphere 
of  the  planets  or  by  reflection  at  their  surfaces. 

Moon. — On  the  moon  the  results  of  our  observations 
have  been  negative.  The  spectra  of  the  various  parts  of 
the  moon's  surface,  when  examined  under  different  condi- 
tions of  illumination,  showed  no  indication  of  an  atmos- 
phere about  the  moon.  I  also  watched  the  spectrum  of  a 


372  HUGGINS 

star  as  the  dark  edge  of  the  moon  advanced  toward  the 
star  and  then  occulted  it.  No  signs  of  a  lunar  atmosphere 
presented  themselves. 

Jupiter. — In  the  spectrum  of  Jupiter  lines  are  seen  which 
indicate  the  existence  of  an  absorptive  atmosphere  about 
this  planet.  In  this  diagram  these  lines  are  presented  as 
they  appeared  when  viewed  simultaneously  with  the  spec- 
trum of  the  sky,  which,  at  the  time  of  observation,  reflected 
the  light  of  the  setting  sun.  One  strong  band  corresponds 
with  some  terrestrial  atmospheric  lines,  and  probably  indi- 
cates the  presence  of  vapours  similar  to  those  which  are 
about  the  earth.  Another  band  has  no  counterpart  among 
the  lines  of  absorption  of  our  atmosphere,  and  tells  us  of 
some  gas  or  vapour  which  does  not  exist  in  the  earth's 
atmosphere. 

Saturn. — The  spectrum  of  Saturn  is  feeble,  but  lines 
similar  to  those  which  distinguish  the  spectrum  of  Jupiter 
were  detected.  These  lines  are  less  strongly  marked  in  the 
ansae  of  the  rings,  and  show  that  the  absorptive  power  of 
the  atmosphere  about  the  rings  is  less  than  that  of  the 
atmosphere  which  surrounds  the  ball.  A  distinguished  for- 
eigner present  at  the  meeting,  Janssen,  has  quite  recently 
found  that  several  of  the  atmospheric  lines  in  this  part  of 
the  spectrum  are  produced  by  aqueous  vapour.  It  appears 
to  be  very  probable  that  aqueous  vapour  exists  in  the 
atmospheres  of  Jupiter  and  Saturn. 

Mars. — On  one  occasion  some  remarkable  groups  of 
lines  were  seen  in  the  more  refrangible  part  of  the  spectrum 
of  Mars.  These  may  be  connected  with  the  source  of  the 
red  colour  which  distinguishes  this  planet. 

Venus. — Though  the  spectrum  of  Venus  is  brilliant, 
and  the  lines  of  Fraunhofer  were  well  seen,  no  additional 
lines  affording  evidence  of  an  atmosphere  about  Venus 
were  detected.  The  absence  of  lines  may  be  due  to 
the  circumstance  that  the  light  is  probably  reflected, 
not  from  the  planetary  surface,  but  from  clouds  at  some 
elevation  above  it.  The  light  which  reaches  us  in  this 
way  by  reflection  from  clouds  would  not  have  been  ex- 


SPECTRUM  ANALYSIS  373 

posed  to  the  absorbent  action  of  the  lower  and  denser 
strata  of  the  planet's  atmosphere. 

THE  FIXED   STARS 

The  fixed  stars,  though  immensely  more  remote  and 
less  conspicuous  in  brightness  than  the  moon  and  planets, 
yet,  because  they  are  original  sources  of  light,  furnish  us 
with  fuller  indications  of  their  nature. 

To  each  succeeding  age  the  stars  have  been  a  beauty 
and  a  mystery.  Not  only  children,  but  the  most  thought- 
ful of  men,  often  repeat  the  sentiment  expressed  in  the 
well-known  lines: 

"Twinkle,  twinkle,  little  star; 
How  I  wonder  what  you  are!  " 

The  telescope  was  appealed  to  in  vain,  for  in  the  largest  in- 
struments the  stars  remain  diskless — brilliant  points  merely. 

The  stars  have  indeed  been  represented  as  suns,  each 
upholding  a  dependent  family  of  planets.  This  opinion 
rested  upon  a  possible  analogy  alone.  It  was  not  more 
than  a  speculation.  We  possessed  no  certain  knowledge 
from  observation  of  the  true  nature  of  those  remote  points 
of  light.  This  long  and  earnestly  coveted  information  is 
at  last  furnished  by  spectrum  analysis.  We  are  now  able 
to  read  in  the  light  of  each  star  some  indications  of  its 
nature.  Since  I  have  not  a  magician's  power  to  convert 
this  theatre  into  an  observatory,  and  so  exhibit  to  you  the 
spectra  of  the  stars  themselves,  I  have  provided  photo- 
graphs of  careful  drawings.  These  photographs  Mr.  Ladd 
will  exhibit  upon  the  screen  by  means  of  the  electric  lamp. 
I  will  take  first  the  spectra  of  two  bright  stars  which  we 
have  examined  with  great  care. 

The  upper  one  represents  the  spectrum  of  Aldebaran, 
and  the  other  that  of  Betelgeux,  the  star  marked  a  in  the 
constellation  of  Orion. 

The  positions  of  all  these  dark  lines,  about  eighty  in  each 
star,  were  determined  by  careful  and  repeated  measures. 
These  measured  lines  form  but  a  small  part  of  the  numer- 


374  HUGGINS 

ous  fine  lines  which  may  be  seen  in  the  spectra  of  these 
stars. 

Beneath  the  spectrum  of  each  star  are  represented  the 
bright  lines  of  the  metals  which  have  been  compared  with 
it.  These  terrestrial  spectra  appeared  in  the  instrument  as 
you  now  see  them  upon  the  screen,  in  juxtaposition  with 
the  spectrum  of  the  star.  By  such  an  arrangement  it  is  pos- 
sible to  determine  with  great  accuracy  whether  or  not  any 
of  these  bright  lines  actually  coincide  with  any  of  the  dark 
ones.  For  example: 

This  closely  double  line  is  characteristic  of  sodium. 
You  see  that  it  coincides,  line  for  line,  with  a  dark  line  simi- 
larly double  in  the  star.  The  vapour  of  sodium  is  therefore 
present  in  the  atmosphere  of  the  star,  and  sodium  forms 
one  of  the  elements  of  the  matter  of  this  brilliant  but  re- 
mote star. 

These  three  lines  in  the  green  are  produced,  so  far  as 
we  know,  by  the  luminous  vapour  of  magnesium  alone. 
These  lines  agree  in  position  exactly  line  for  line  with  three 
dark  stellar  lines.  The  conclusion,  therefore,  appears  well 
founded  that  another  of  the  constituents  of  this  star  is  mag- 
nesium. 

Again,  there  are  two  strong  lines  peculiar  to  the  ele- 
ment hydrogen;  one  line  has  its  place  in  the  red  part  of  the 
spectrum,  the  other  at  the  blue  limit  of  the  green.  Both 
of  these  correspond  to  dark  lines  of  absorption  in  the 
spectrum  of  the  star.  Hydrogen,  therefore,  is  present  in 
the  star. 

In  a  similar  way,  other  elements,  among  them  bismuth, 
antimony,  telurium,  and  mercury,  have  been  shown  to  exist 
in  the  star. 

Now,  in  reference  to  all  those  elements,  the  evidence 
does  not  rest  upon  the  coincidence  of  one  line,  which  would 
be  worth  but  little,  but  upon  the  coincidence  of  a  group 
of  two,  three,  or  four  lines,  occurring  in  different  parts  of 
the  spectrum.  Other  corresponding  lines  are  probably  also 
present,  but  the  faintness  of  the  star's  light  limited  our 
comparisons  to  the  stronger  lines  of  each  element. 


SPECTRUM  ANALYSIS 


375 


What  elements  do  the  numerous  other  lines  in  the  star 
represent?  Some  of  them  are  probably  due  to  the  vapours 
of  other  terrestrial  elements,  which  we  have  not  yet  com- 
pared with  these  stars.  But  may  not  some  of  these  lines  be 
the  signs  of  primary  forms  of  matter  unknown  upon  the 
earth?  Elements  new  to  us  may  here  show  themselves 
which  form  large  and  important  series  of  compounds,  and 
therefore  give  a  special  character  to  the  physical  conditions 
of  these  remote  systems.  In  a  similar  manner  the  spectra 
of  terrestrial  substances  have  been  compared  with  several 
other  stars.  The  results  are  given  in  the  diagrams.  Five 
or  six  elements  have  been  detected  in  Betelgeux.  Ten 
other  elements  do  not  appear  to  have  a  place  in  the  consti- 
tution of  this  star. 

ft  Pegasi  contains  sodium,  magnesium,  and  perhaps 
barium. 

Sirius  contains  sodium,  magnesium,  iron,  and  hy- 
drogen. 

a  Lyrae  (Vega)  contains  sodium,  magnesium,  and  iron. 

Pollux  contains  sodium,  magnesium,  and  iron. 

About  sixty  other  stars  have  been  examined,  all  of 
which  appear  to  have  some  elements  in  common  with  the 
sun  and  earth,  but  the  selective  grouping  of  the  elements 
in  each  star  is  probably  peculiar  and  unique. 

A  few  stars,  however,  stand  out  from  the  rest,  and 
appear  to  be  characterized  by  a  peculiarity  of  great  sig- 
nificance. These  stars  are  represented  by  Betelgeux  and 
ft  Pegasi.  The  general  grouping  of  the  lines  of  absorption 
in  these  stars  is  peculiar,  but  the  remarkable  and  excep- 
tional feature  of  their  spectra  is  the  absence  of  the  two 
lines  which  indicate  hydrogen,  one  line  in  the  red  and  the 
other  in  the  green.  These  lines  correspond  to  Fraunhofer's 
C  and  F.  The  absence  of  these  lines  in  some  stars  shows 
that  the  lines  C  and  F  are  not  due  to  the  aqueous  vapour 
of  the  atmosphere. 

We  hardly  venture  to  suggest  that  the  planets  which 
may  surround  these  suns  probably  resemble  them  in  not 
possessing  the  important  element  hydrogen.  To  what 


3/6 


MUGGINS 


forms  of  life  could  such  planets  be  adapted?  Worlds  with- 
out water!  A  power  of  imagination  like  that  possessed  by 
Dante  would  be  needed  to  people  such  planets  with  living 
creatures. 

It  is  worthy  of  consideration  that,  with  these  few  ex- 
ceptions, the  terrestrial  elements  which  appear  most  widely 
diffused  through  the  host  of  stars  are  precisely  some  of 
those  which  are  essential  to  life  such  as  it  exists  upon  the 
earth — namely,  hydrogen,  sodium,  magnesium,  and  iron. 
Besides,  hydrogen,  sodium,  and  magnesium  represent  the 
ocean,  which  is  an  essential  part  of  a  world  constituted  like 
the  earth. 

We  learn  from  these  observations  that  in  plan  of  struc- 
ture the  stars,  or  at  least  the  brightest  of  them,  resemble  the 
sun.  Their  light,  like  that  of  the  sun,  emanates  from  in- 
tensely white-hot  matter,  and  passes  through  an  atmosphere 
of  absorbent  vapours.  With  this  unity  of  general  plan  of 
structure  there  exists  a  great  diversity  among  the  individual 
stars.  Star  differs  from  star  in  chemical  constitution.  May 
we  not  believe  that  the  individual  peculiarities  of  each  star 
are  essentially  connected  with  the  special  purpose  which  it 
subserves,  and  with  the  living  beings  which  may  inhabit 
the  planetary  worlds  by  which  it  may  possibly  be  sur- 
rounded? 

When  we  had  obtained  this  new  information  respecting 
the  true  nature  of  the  stars,  our  attention  was  directed  to 
the  phenomena  which  specially  distinguish  some  of  the 
stars. 

COLOURS  OF  THE  STARS 

When  the  air  is  clear,  especially  in  southern  climes,  the 
twinkling  stars  do  not  all  resemble  diamonds;  here  and 
there  may  be  seen  in  beauteous  contrast  richly  coloured 
gems. 

The  colour  of  the  light  of  the  stars  which  are  bright  to 
the  naked  eye  is  always  some  tint  of  red,  orange,  or  yellow. 
When,  however,  a  telescope  is  employed,  in  close  compan- 
ionship with  many  of  these  ruddy  and  orange  stars,  other 


ANALYSIS  377 

fainter  stars  become  visible,  the  colour  of  which  may  be 
blue,  or  green,  or  purple. 

Now  it  appeared  to  us  to  be  probable  that  the  origin 
of  these  differences  of  colour  among  the  stars  may  be  indi- 
cated by  their  spectra, 

Since  we  had  found  that  the  source  of  the  light  of  the 
stars  is  incandescent  solid  or  liquid  matter,  it  appeared  to 
be  very  probable  that  at  the  time  of  its  emission  the  light  of 
all  the  stars  is  white  alike.  The  colours  observed  among 
them  must  then  be  caused  by  some  modification  suffered 
by  the  light  after  its  emission. 

Again,  it  was  obvious  that  if  the  dark  lines  of  absorp- 
tion were  more  numerous  or  stronger  in  some  part  of  the 
spectrum,  then  those  colours  would  be  subdued  in  power, 
relatively  to  the  colour  in  which  few  lines  only  occur. 
These  latter  colours,  remaining  strong,  would  predominate, 
and  give  to  the  light,  originally  white,  their  own  tints. 
These  suppositions  have  been  confirmed  by  observations. 

Mr.  Ladd  will  throw  upon  the  screen  the  spectrum  of 
Sirius,  which  may  be  taken  as  an  illustration  of  the  stars  the 
light  of  which  is  white. 

As  might  be  expected,  the  spectra  of  these  stars  are  re- 
markable for  their  freedom  from  strong  groups  of  absorp- 
tion lines.  The  dark  lines,  though  present  in  great  number, 
are  all,  with  one  exception,  very  thin  and  faint,  and  too 
feeble  to  modify  the  original  whiteness  of  the  light.  The 
one  exception  consists  of  three  very  strong  single  lines: 
one  line  corresponding  to  Fraunhofer's  C,  one  to  F,  and 
the  other  near  G.  Two  of  these  certainly  indicate  the  pres- 
ence of  hydrogen.  This  peculiarity,  which  seems  invariably 
connected  with  colourless  stars,  is  very  suggestive,  and  in- 
vites speculation.  May  it  be  a  sign  of  a  temperature  of 
extreme  fierceness? 

Let  us  now  examine  the  spectrum  of  an  orange  star. 

This  diagram  represents  the  spectrum  of  the  brighter 
of  the  two  stars  which  form  the  double  star  a  Herculis.  In 
the  spectrum  of  this  star  the  green  and  blue  parts  of  the 
light,  and  also  the  deep  red,  are  subdued  with  strong  groups 


378  HUGGINS 

of  lines,  while  the  orange  and  yellow  rays  preserve  nearly 
their  original  intensity,  and  therefore  predominate  in  the 
star's  light. 

The  question  yet  remained  to  be  answered,  Would  the 
faint  telescopic  stars,  which  are  blue,  green,  and  purple,  and 
which  are  never  found  alone  in  the  heavens,  but  always 
under  the  protection  of  a  strong  ruddy  or  orange  star,  fur- 
nish spectra  in  accordance  with  this  theory? 

With  some  little  difficulty,  and  by  means  of  a  special 
arrangement  of  the  spectrum  apparatus,  we  succeeded  in 
observing  the  spectra  of  the  components  of  some  double 
stars.  There  will  now  be  thrown  upon  the  screen  the  well- 
known  double  star  £  Cygni.  In  a  large  telescope  the 
colours  of  the  two  stars  are  beautifully  contrasted,  as  they 
now  appear  upon  the  screen.  The  spectra  of  these  stars 
are  now  shown.  The  upper  spectrum  represents  the  orange 
star,  the  lower  one  that  of  its  beautiful  blue  but  feeble  com- 
panion. In  the  orange  star  you  observe  that  the  dark  lines 
are  strongest  and  most  closely  grouped  in  the  blue  and 
violet  parts  of  the  spectrum,  and  the  orange  rays  therefore, 
which  are  comparatively  free  from  lines,  predominate. 

In  the  delicate  blue  companion,  the  strongest  groups  of 
lines  are  found  in  the  yellow,  orange,  and  in  part  of  the 
red.  In  the  arrangement  of  these  groups  of  lines  we  have 
a  sufficient  cause  for  the  predominance  of  the  other  por- 
tions of  the  spectrum  which  unite  in  the  eye  to  give  the 
blue  purple  colour  of  the  light  of  this  star. 

We  have,  therefore,  shown  that  the  colours  of  the  stars 
are  produced  by  the  vapours  existing  in  their  atmosphere. 
The  chemical  constitution  of  a  star's  atmosphere  will  de- 
pend upon  the  elements  existing  in  the  star  and  upon  its 
temperature. 

VARIABLE  STARS 

The  brightness  of  many  of  the  stars  is  found  to  be 
variable.  From  night  to  night,  from  month  to  month,  or 
from  season  to  season,  their  light  may  be  observed  to  be 
continually  changing,  at  one  time  increasing,  at  another 


SPECTRUM  ANALYSIS  379 

time  diminishing.  The  careful  study  of  these  variable  stars 
by  numerous  observers  has  shown  that  their  continual 
changes  do  not  take  place  in  an  uncertain  or  irregular  man- 
ner. The  greater  part  of  these  remarkable  objects  wax  and 
wane  in  accordance  with  a  fixed  law  of  periodic  variations 
which  is  peculiar  to  each. 

We  have  been  seeking  for  some  time  to  throw  light 
upon  this  strange  phenomenon  by  means  of  observation  of 
their  spectra.  If  in  any  case  the  periodic  variation  of  bright- 
ness is  associated  with  physical  changes  occurring  in  the 
star,  we  might  obtain  some  information  by  means  of  the 
prism.  Again,  if  the  diminution  in  brightness  of  a  star 
should  be  caused  by  the  interposition  of  a  dark  body,  then, 
in  that  case,  if  the  dark  body  be  surrounded  with  an  atmos- 
phere, its  presence  might  possibly  be  revealed  to  us  by  the 
appearance  of  additional  lines  of  absorption  in  the  spectrum 
of  the  star  when  at  its  minimum.  One  such  change  in  the 
spectrum  of  a  variable  star  we  believe  we  have  already  ob- 
served. 

Betelgeux  isva  star  of  a  moderate  degree  of  variability. 
When  this  star  was  at  its  maximum  brilliancy,  in  February 
last,  we  missed  a  group  of  lines,  the  exact  position  of  which 
we  had  determined  with  great  accuracy  by  micrometric 
measurement  some  two  years  before. 

We  have  observed  the  spectra  of  several  variable  stars  at 
different  phases  of  their  periodic  variation,  but  our  results 
are  not  yet  complete. 

It  is  worthy  of  notice  that  the  variable  stars  which  have 
a  ruddy  or  an  orange  tint  possess  spectra  analogous  to  that 
of  Betelgeux  and  ft  Pegasi. 

As  an  example  of  this  group  of  variable  stars,  Mr.  Ladd 
will  throw  upon  the  screen  the  spectrum  of  /*  Cephei  when 
at  its  maximum. 

TEMPORARY   STARS 

With  the  variable  stars  modern  opinion  would  associate 
the  remarkable  phenomena  of  the  so-called  new  stars  which 
occasionally,  but  at  long  intervals,  have  suddenly  appeared 


380  HUGGINS 

in  the  sky.  But  in  no  case  has  a  permanently  bright  star 
been  added  to  the  heavens.  The  splendour  of  all  these 
objects  was  temporary  only,  though  whether  they  died 
out  or  still  exist  as  extremely  faint  stars  is  uncertain.  In 
case  of  the  two  modern  temporary  stars,  that  seen  by 
Mr.  Hind  in  1845,  an<3  the  bright  star  recently  -observed 
in  Corona,  though  they  have  lost  their  ephemeral  glory, 
they  still  continue  as  stars  of  the  tenth  and  eleventh  mag- 
nitudes. 

The  old  theories  respecting  these  strange  objects  must 
be  rejected.  We  can  not  believe  with  Tycho  Brahe  that 
objects  so  ephemeral  are  new  creations,  nor  with  Riccioli 
that  they  are  stars  brilliant  on  one  side  only,  which  have 
been  suddenly  turned  round  by  the  Deity.  The  theory  that 
they  have  suddenly  darted  toward  us  with  a  velocity  greater 
than  that  of  light,  from  a  region  of  remote  invisibility,  will 
not  now  find  supporters. 

On  the  1 2th  of  May  last  a  star  of  the  second  magnitude 
suddenly  burst  forth  in  the  constellation  of  the  Northern 
Crown.  Thanks  to  the  kindness  of  the  discoverer  of  this 
phenomenon,  Mr.  Birmingham,  of  Tuam,  I  was  enabled, 
conjointly  with  Dr.  Miller,  to  examine  the  spectrum  of 
this  star  on  the  i6th  of  May,  when  it  had  not  fallen  much 
below  the  third  magnitude. 

I  ought  to  state  that  Mr.  Barker,  of  London,  Canada 
West,  who  announced  an  observation  of  this  star  on  the 
1 4th  of  May  in  the  "  Canadian  Free  Press,"  now  claims 
to  have  seen  the  star  on  May  4th,  and  states  that  it  in- 
creased in  brilliancy  up  to  May  loth,  when  it  was  at  its 
maximum. 

The  spectrum  of  this  star  consists  of  two  distinct  spectra. 
One  of  these  is  formed  by  these  four  bright  lines.  The 
other  spectrum  is  analogous  to  the  spectra  of  the  sun  and 
stars. 

These  two  spectra  represent  two  distinct  sources  of 
light.  Each  spectrum  is  formed  by  the  decomposition  of 
light  which  is  independent  of  the  light  which  gives  birth  to 
the  other  spectrum. 


SPECTRUM   ANALYSIS  381 

The  continuous  spectrum,  crowded  with  groups  of  dark 
lines,  shows  that  there  exists  a  photosphere  of  incandescent 
solid  or  liquid  matter.  Further,  that  there  is  an  atmos- 
phere of  cooler  vapours,  which  give  rise  by  absorption  to 
the  groups  of  dark  lines. 

So  far  the  constitution  of  this  object  is  analogous  to  that 
of  the  sun  and  stars;  but  in  addition  there  is  the  second 
spectrum,  which  consists  of  bright  lines.  There  is,  there- 
fore, a  second  and  distinct  source  of  light,  and  this  must 
be,  as  the  character  of  the  spectrum  shows,  luminous  gas. 
Now  the  position  of  the  two  principal  bright  lines  of 
this  spectrum  informs  us  that  one  of  the  luminous  gases 
is  hydrogen.  The  great  brightness  of  these  lines  shows 
that  the  luminous  gas  is  hotter  than  the  photosphere. 
These  facts,  taken  in  connection  writh  the  suddenness  of  the 
outburst  of  light  in  the  star,  and  its  immediate  very  rapid 
decline  in  brightness  from  the  second  magnitude  down  to 
the  eighth  magnitude  in  twelve  days,  suggested  to  us  the 
startling  speculation  that  the  star  had  become  suddenly 
inwrapped  in  the  flames  of  burning  hydrogen.  In  conse- 
quence, it  may  be,  of  some  great  convulsion,  enormous 
quantities  of  gas  were  set  free.  A  large  part  of  this  gas  con- 
sisted of  hydrogen,  which  was  burning  about  the  star  in 
combination  with  some  other  element.  This  flaming  gas 
emitted  the  light  represented  by  the  spectrum  of  bright 
lines.  The  increased  brightness  of  the  spectrum  of  the 
other  part  of  the  star's  light  may  show  that  this  fierce  gase- 
ous conflagration  had  heated  to  a  more  vivid  incandescence 
the  solid  matter  of  the  photosphere.  As  the  free  hydrogen 
became  exhausted  the  flames  gradually  abated,  the  photo- 
sphere became  less  vivid,  and  the  star  waned  down  to  its 
former  brightness. 

We  must  not  forget  that  light,  though  a  swift  mes- 
senger, requires  time  to  pass  from  the  star  to  us.  The  great 
physical  convulsion  which  is  new  to  us  is  already  an  event 
of  the  past  with  respect  to  the  star  itself.  For  years  the  star 
has  existed  under  the  new  conditions  which  followed  this 
fiery  catastrophe. 


382  HUGGINS 


NEBULAE 

I  now  pass  to  objects  of  another  order. 

When  the  eye  is  aided  by  a  telescope  of  even  moderate 
power,  a  large  number  of  faintly  luminous  patches  and 
spots  come  forth  from  the  darkness  of  the  sky,  which  are 
in  strong  contrast  with  the  brilliant  but  pointlike  images 
of  the  stars.  A  few  of  these  objects  may  be  easily  discerned 
to  consist  of  very  faint  stars  closely  aggregated  together. 
Many  of  these  strange  objects  remain,  even  in  the  largest 
telescopes,  unresolved  into  stars,  and  resemble  feebly  shin- 
ing clouds,  or  masses  of  phosphorescent  haze.  During  the 
last  one  hundred  and  fifty  years  the  intensely  important 
question  has  been  continually  before  the  mind  of  astron- 
omers, "  What  is  the  true  nature  of  these  faint,  cometlike 
masses?  " 

The  interest  connected  with  an  answer  to  this  question 
has  much  increased  since  Sir  William  Herschel  suggested 
that  these  objects  are  portions  of  the  primordial  material 
out  of  which  the  existing  stars  have  been  fashioned;  and 
further,  that  in  these  objects  we  may  study  some  of  the 
stages  through  which  the  suns  and  planets  pass  in  their 
development  from  luminous  cloud. 

The  telescope  has  failed  to  give  any  certain  information 
of  the  nature  of  the  nebulae.  It  is  true  that  each  successive 
increase  of  aperture  has  resolved  more  of  these  objects  into 
bright  points,  but  at  the  same  time  other  fainter  nebulae 
have  been  brought  into  view,  and  fantastic  wisps  and  dif- 
fused patches  of  light  have  been  seen,  which  the  mind 
almost  refuses  to  believe  can  be  due  to  the  united  glare  of 
innumerable  suns  still  more  remote. 

Spectrum  analysis,  if  it  could  be  successfully  applied 
to  objects  so  excessively  faint,  was  obviously  a  method  of 
investigation  specially  suitable  for  determining  whether 
any  essential  physical  distinction  separates  the  nebulae  from 
the  stars. 

I  selected  for  the  first  attempt,  in  August,  1864,  one  of 
the  class  of  small  but  comparatively  bright  nebulae.  My 


SPECTRUM  ANALYSIS  383 

surprise  was  very  great  on  looking  into  the  small  telescope 
of  the  spectrum  apparatus  to  perceive  that  there  was  no 
appearance  of  a  band  of  coloured  light,  such  as  a  star  would 
give,  but  in  place  of  this  there  were  three  isolated  bright 
lines  only. 

This  observation  was  sufficient  to  solve  the  long-agi- 
tated inquiry  in  reference  to  this  object  at  least,  and  to  show 
that  it  was  not  a  group  of  stars,  but  a  true  nebula. 

A  spectrum  of  this  character,  so  far  as  our  knowledge 
at  present  extends,  can  be  produced  only  by  light  which 
has  emanated  from  matter  in  the  state  of  gas.  The  light 
of  this  nebula,  therefore,  was  not  emitted  from  incandescent 
solid  or  liquid  matter,  as  is  the  light  of  the  sun  and  stars, 
but  from  glowing  or  luminous  gas. 

It  was  of  importance  to  learn,  if  possible,  from  the  posi- 
tion of  these  bright  lines,  the  chemical  nature  of  the  gas 
or  gases  of  which  this  nebula  consists. 

Measures  taken  by  the  micrometer  of  the  most  brilliant 
of  the  bright  lines  showed  that  this  line  occurs  in  the  spec- 
trum very  nearly  in  the  position  of  the  brightest  of  the 
lines  in  the  spectrum  of  nitrogen.  The  experiment  was 
then  made  of  comparing  the  spectrum  of  nitrogen  directly 
with  the  bright  lines  of  the  nebula.  I  found  that  the  bright- 
est of  the  lines  of  the  nebula  coincided  with  the  strongest 
of  the  group  of  lines  which  are  peculiar  to  nitrogen.  It 
may  be,  therefore,  that  the  occurrence  of  this  one  line  only 
indicates  a  form  of  matter  more  elementary  than  nitrogen, 
and  which  our  analysis  has  not  yet  enabled  us  to  detect. 

In  a  similar  manner  the  faintest  of  the  lines  was  found 
to  coincide  with  the  green  line  of  hydrogen. 

The  middle  line  of  the  three  lines  which  form  the  spec- 
trum of  the  nebula  does  not  coincide  with  any  strong  line 
in  the  spectra  of  about  thirty  of  the  terrestrial  elements.  It 
is  not  far  from  the  line  of  barium,  but  it  does  not  coincide 
with  it.  Besides  these  bright  lines,  there  was  also  an  ex- 
ceedingly faint  continuous  spectrum.  The  spectrum  had 
no  apparent  breadth,  and  must,  therefore,  have  been  formed 
by  a  minute  point  of  light.  The  position  of  this  faint  spec- 


384  HUGGINS 

trum,  which  crossed  the  bright  lines  about  the  middle  of 
their  length,  showed  that  the  bright  point  producing  it  was 
situated  about  the  centre  of  the  nebula.  Now,  this  nebula 
possesses  a  minute  but  bright  nucleus.  We  learn  from  this 
observation  that  the  matter  of  the  nucleus  is  almost  cer- 
tainly not  in  a  state  of  gas,  as  is  the  material  of  the  sur- 
rounding nebula.  It  consists  of  opaque  matter,  which  may 
exist  in  the  form  of  an  incandescent  fog  of  solid  or  liquid 
particles. 

The  new  and  unexpected  results  arrived  at  by  the  pris- 
matic examination  of  this  nebula  showed  the  importance  of 
examining  as  many  as  possible  of  these  remarkable  bodies. 
Would  all  the  nebulae  give  similar  spectra?  Especially  it 
was  of  importance  to  ascertain  whether  those  nebulae  which 
the  telescope  had  certainly  resolved  into  a  close  aggre- 
gation of  bright  points  would  give  a  spectrum  indicating 
gaseity. 

The  observation  with  the  prism  of  these  objects  is  ex- 
tremely difficult,  on  account  of  their  great  faintness.  Be- 
sides this,  it  is  only  when  the  sky  is  very  clear  and  the  moon 
is  absent  that  the  prismatic  arrangement  of  their  light  is 
even  possible.  During  the  last  two  years  I  have  examined 
the  spectra  of  more  than  sixty  nebulae  and  clusters.  These 
may  be  divided  into  two  great  groups.  One  group  con- 
sists of  the  nebulae  which  give  a  spectrum  similar  to  the  one 
I  have  already  described,  or  else  of  one  or  two  only  of  the 
three  bright  lines.  Of  the  sixty  objects  examined,  about 
one  third  belong  to  the  class  of  gaseous  bodies.  The  light 
from  the  remaining  forty  nebulae  and  clusters  becomes 
spread  out  by  the  prism  into  a  spectrum  which  is  appar- 
ently continuous. 

I  will  exhibit  upon  the  screen  a  few  of  the  more  remark- 
able of  the  nebulae  which  are  gaseous  in  their  constitution. 

This  photograph  is  from  a  drawing  by  Lord  Rosse  of 
a  small  nebula  in  Aquarius.  (I.  H.  IV.) 

We  have  here  a  gaseous  system  which  reminds  the 
observer  of  Saturn  and  his  rings.  The  ring  is  seen  edge- 
wise. 


SPECTRUM   ANALYSIS  385 

The  three  bright  lines  represent  the  spectrum  into  which 
the  light  of  this  object  is  resolved  by  the  prism. 

In  this  other  nebula  we  find  probably  an  analogous 
general  form  of  structure.  In  consequence  of  the  nebula 
lying  in  a  different  position  to  us,  its  ring  is  seen,  not  edge- 
wise, but  open  on  the  flat.  The  spectrum  consists  of  three 
bright  lines. 

The  arrangement  of  the  streams  of  light  in  the  object 
now  on  the  screen  suggests  a  spiral  structure.  This  nebula 
is  remarkable  as  the  only  one  in  which,  in  addition  to  the 
three  bright  lines,  a  fourth  line  was  also  seen. 

The  most  remarkable,  and  possibly  the  nearest  to  our 
system,  of  the  nebulae  presenting  a  ring  formation,  is  the 
well-known  annular  nebula  in  Lyra.  The  spectrum  con- 
sists of  one  bright  line  only.  When  the  slit  of  the  instru- 
ment crosses  the  nebula,  the  line  consists  of  two  brighter 
portions,  corresponding  to  the  sections  of  the  ring.  A 
much  fainter  line  joins  them,  which  shows  that  the  faint 
central  portion  of  the  nebula  has  a  similar  constitution. 

A  nebula  remarkable  for  its  large  extent  and  peculiar 
form  is  that  known  as  the  dumb-bell  nebula.  The  spectrum 
of  this  nebula  consists  of  one  line  only.  A  prismatic  exam- 
ination of  the  light  from  different  parts  of  this  object  shows 
that  it  is  throughout  of  a  similar  constitution. 

The  most  widely  known,  perhaps,  of  all  the  nebulae  is  the 
remarkable  cloudlike  object  in  the  sword-handle  of  Orion. 

This  object  is  also  gaseous.  Its  spectrum  consists  of 
three  bright  lines.  Lord  Rosse  informs  me  that  the  bluish- 
green  matter  of  the  nebula  has  not  been  resolved  by  his 
telescope.  In  some  parts,  however,  he  sees  a  large  number 
of  very  minute  red  stars,  which,  though  apparently  con- 
nected with  the  irresolvable  matter  of  the  nebula,  are  yet 
doubtless  distinct  from  it.  These  stars  would  be  too  faint 
to  furnish  a  visible  spectrum. 

I  now  pass  to  some  examples  of  the  other  great  group 
of  nebulae  and  clusters. 

All  the  true  clusters,  which  are  resolved  by  the  telescope 
into  distinct  bright  points,  give  a  spectrum,  which  does  not 
25 


386 


HUGGINS 


consist  of  separate  bright  lines,  but  is  apparently  continu- 
ous in  its  light.  There  are  many  nebulae  which  furnish  a 
similar  spectrum. 

I  take,  as  an  example  of  these  nebulae,  the  great  nebula 
in  Andromeda,  which  is  visible  to  the  naked  eye,  and  is  not 
seldom  mistaken  for  a  comet.  The  spectrum  of  this  nebula, 
though  apparently  continuous,  has  some  suggestive  peculi- 
arities. The  whole  of  the  red  and  part  of  the  orange  are 
wanting.  Besides  this  character,  the  brighter  parts  of  the 
spectrum  have  a  very  unequal  and  mottled  appearance. 

It  is  remarkable  that  the  easily  resolved  cluster  in  Her- 
cules has  a  spectrum  precisely  similar.  The  prismatic  con- 
nection of  this  cluster  with  the  nebula  in  Andromeda  is  con- 
firmed by  telescopic  observation.  Lord  Rosse  has  dis- 
covered in  this  cluster  dark  streaks  or  lines  similar  to  those 
which  are  seen  in  the  nebula  in  Andromeda. 

In  connection  with  these  observations,  it  was  of  great 
interest  to  ascertain  whether  the  broad  classification 
afforded  by  the  prism  of  the  nebulae  and  clusters  would  cor- 
respond with  the  indications  of  resolvability  furnished  by 
the  telescope.  Would  it  be  found  that  all  the  unresolved 
nebulae  are  gaseous,  and  that  those  which  gave  a  continuous 
spectrum  are  clusters  of  stars? 

Lord  Oxmantown  has  examined  all  the  observations  of 
the  sixty  nebulae  and  clusters  in  my  list,  which  have  been 
made  with  the  great  reflecting  telescope  erected  by  his 
father,  the  Earl  of  Rosse. 

The  results  are  given  in  this  table: 


Continuous 
spectrum. 

Gaseous 
spectrum. 

Clusters    ...      .                  .  .  .  .    ....... 

IO 

o 

o 

IO 

6 

Blue  or  green,  no  resolvability,  no  resolvability  seen  .  -I 

0 

6 

4 
5 

Not  observed  by  Lord  Rosse  

3i 

IO 

15 

4" 

41 

iQ 

SPECTRUM   ANALYSIS  387 

Considering  the  great  difficulty  of  successful  telescopic 
observation  of  these  objects,  the  correspondence  between 
the  results  of  prismatic  and  telescopic  observation  may  be 
regarded  as  close  and  suggestive. 

Half  of  the  nebulae  which  give  a  continuous  spectrum 
have  been  resolved,  and  about  one  third  more  are  probably 
resolvable;  while  of  the  gaseous  nebulae  none  have  been 
certainly  resolved,  according  to  Lord  Rosse. 

The  inquiry  now  presents  itself  upon  us,  What  super- 
structure of  interpretation  have  we  a  right  to  raise  upon  the 
new  facts  with  which  the  prism  has  furnished  us? 

Is  the  existence  of  the  gaseous  nebulae  an  evidence  of 
the  reality  of  that  primordial  nebulous  matter  required  by 
the  theories  of  Sir  William  Herschel  and  Laplace? 

Again,  if  we  do  not  accept  the  view  that  these  nebulae 
are  composed  of  portions  of  the  original  elementary  mat- 
ter out  of  which  suns  and  planets  have  been  elaborated, 
what  is  the  cosmical  rank  and  relation  which  we  ought  to 
assign  to  them? 

As  aids  to  a  future  determination  of  these  great  ques- 
tions I  will  refer  in  a  few  words  to  some  other  observations. 

COMETS 

There  are  objects  in  the  heavens  which  occasionally, 
and  under  some  conditions,  resemble  closely  some  of  the 
nebulae.  In  some  positions  in  their  orbits  some  of  the 
comets  appear  as  round  vaporous  masses,  and,  except  by 
their  motion,  can  not  be  distinguished  from  nebulae.  Does 
this  occasional  general  resemblance  indicate  a  similarity  of 
nature?  If  such  be  the  case,  if  the  material  of  the  comets 
is  similar  to  that  of  the  nebulae,  then  the  study  of  the  won- 
derful changes  which  comets  undergo  in  the  neighbour- 
hood of  the  sun  may  furnish  useful  information  for  a  more 
correct  interpretation  of  the  structure  and  condition  of  the 
nebulae.  In  1864  Donati  found  that  the  spectrum  of  a 
comet  visible  in  that  year  consisted  of  bright  lines. 

Last  January  a  small  telescopic  comet  was  visible.  Its 
appearance  in  a  large  telescope  is  represented  on  the  screen. 


388  HUGGINS 

It  was  a  nearly  circular,  very  faint  vaporous  mass.  Nearly 
in  the  centre  a  small  and  rather  dim  nucleus  was  seen. 
When  this  object  was  viewed  in  the  spectroscope,  two 
spectra  were  distinguished — a  very  faint  continuous  spec- 
trum of  the  coma  showing  that  it  was  visible  by  reflecting 
solar  light.  About  the  middle  of  this  faint  spectrum  a 
bright  point  was  seen.  This  bright  point  is  the  spectrum 
of  the  nucleus,  and  shows  that  its  light  is  different  from  that 
of  the  coma.  This  short,  bright  line  indicates  that  the 
nucleus  of  this  comet  was  self-luminous,  and  further,  the 
position  of  this  line  of  the  spectrum  suggests  that  the  ma- 
terial of  the  comet  was  similar  to  the  matter  of  which  the 
gaseous  nebulae  consist. 

MEASURES    OF    THE    INTRINSIC    BRIGHTNESS    OF    THE    NEBULAE 

It  appeared  to  me  that  some  information  of  the  nature 
of  the  nebulae  might  be  obtained  from  observations  of  an- 
other order.  If  physical  changes  of  the  magnitude  neces- 
sary for  the  conversion  of  the  gaseous  bodies  into  suns  are 
now  in  progress  in  the  nebulse,  surely  this  process  of  de- 
velopment would  be  accompanied  by  marked  changes  in 
the  intrinsic  brightness  of  their  light,  and  in  their  size. 

Now,  since  the  spectroscope  shows  these  bodies  to  be 
continuous  masses  of  gas,  it  is  possible  to  obtain  an  approxi- 
mate measure  of  their  real  brightness.  It  is  known  that  as 
long  as  a  distant  object  remains  of  sensible  size,  its  bright- 
ness remains  unaltered.  By  a  new  photometric  method  I 
found  the  intrinsic  intensity  of  the  light  of  three  of  the 
gaseous  nebulas  in  terms  of  a  sperm  candle  burning  at  the 
rate  of  158  grains  per  hour: 

Nebula  No.  4,628,  -j-^-g-  part  of  the  intensity  of  the 
candle. 

Annular  nebula,  Lyra,  -^V^  part  of  the  intensity  of  the 
candle. 

Dumb-bell  nebula,  3-5-$-^  part  of  the  intensity  of  the 
candle. 

These  numbers  represent  not  the  apparent  brightness 
only,  but  the  true  brightness  of  these  luminous  masses,  ex- 


SPECTRUM  ANALYSIS  389 

cept  so  far  as  it  may  have  been  diminished  by  a  possible 
power  of  extinction  existing  in  cosmical  space,  and  by 
the  absorption  of  our  atmosphere.  It  is  obvious  that  simi- 
lar observations,  made  at  considerable  intervals  of  time, 
may  show  whether  the  light  of  these  objects  is  undergoing 
increase  or  diminution,  or  is  subject  to  a  periodic  variation. 
If  the  dumb-bell  nebula,  the  feeble  light  of  which  is  not 
more  than  3-5-0  inr  Part  °f tnat  °f  a  candle,  be  in  accordance 
with  popular  theory  a  sun-germ,  then  it  is  scarcely  possible 
to  put  in  an  intelligible  form  the  enormous  number  of  times 
by  which  its  light  must  increase  before  this  faint  nebula, 
feebler  now  in  its  glimmering  than  a  rushlight,  can  rival 
the  dazzling  splendour  of  our  sun. 

MEASURES   OF  THE   NEBULAE 

Some  of  the  nebulae  are  sufficiently  defined  in  outline 
to  admit  of  accurate  measurement.  By  means  of  a  series 
of  micrometric  observations,  it  may  be  possible  to  ascertain 
whether  any  considerable  alteration  in  size  takes  place  in 
nebulae. 

METEORS 

Mr.  Alexander  Herschel  has  recently  succeeded  in  sub- 
jecting another  order  of  the  heavenly  bodies  to  prismatic 
analysis.  He  has  obtained  the  spectrum  of  a  bright  meteor, 
and  also  the  spectra  of  some  of  the  trains  which  meteors 
leave  behind  them.  A  remarkable  result  of  his  observations 
appears  to  be  that  sodium,  in  the  state  of  luminous  vapour, 
is  present  in  the  trains  of  most  meteors. 

CONCLUSION 

In  conclusion,  the  new  knowledge  that  has  been  gained 
from  the  observations  with  the  prism  may  be  summed  up 
as  follows: 

1.  All  the  brighter  stars,  at  least,   have  a  structure 
analogous  to  that  of  the  sun. 

2.  The  stars  contain  material  elements  common  to  the 
sun  and  earth. 

3.  The  colours  of  the  stars  have  their  origin  in  the 


390  MUGGINS 

chemical  constitution  of  the  atmospheres  which  surround 
them. 

4.  The  changes  in  brightness  of  some  of  the  variable 
stars  are  attended  with  changes  in  the  lines  of  absorption  of 
their  spectra. 

5.  The  phenomena  of  the  star  in  Corona  appear  to 
show  that  in  this  object,  at  least,  great  physical  changes 
are  in  operation. 

6.  There  exist  in  the  heavens  true  nebulae.    These  ob- 
jects consist  of  luminous  gas. 

7.  The  material  of  comets  is  very  similar  to  the  matter 
of  the  gaseous  nebulae,  and  may  be  identical  with  it. 

8.  The  bright  point  of  the  star  clusters  may  not  be  in 
all  cases  stars  of  the  same  order  as  the  separate  bright  stars. 

It  may  be  asked  what  cosmical  theory  of  the  origin  and 
relations  of  the  heavenly  bodies  do  these  new  facts  suggest? 
It  would  be  easy  to  speculate,  but  it  appears  to  me  that  it 
would  not  be  philosophical  to  dogmatize  at  present  on  a 
subject  of  which  we  know  so  very  little.  Our  views  of  the 
universe  are  undergoing  important  changes ;  let  us  wait  for 
more  facts,  with  minds  unfettered  by  any  dogmatic  theory, 
and  therefore  free  to  receive  the  obvious  teaching,  what- 
ever it  may  be,  of  new  observations. 

Star  differs  from  star  in  glory,  each  nebula  and  each 
cluster  has  its  own  special  features;  doubtless  in  wisdom, 
and  for  high  and  important  purposes,  the  Creator  has  made 
them  all. 


CELESTIAL   SPECTROSCOPY1 

IT  is  now  many  years  since  this  association  has  done  hon- 
our to  the  science  of  astronomy  in  the  selection  of  its 
president.  Since  Sir  George  Airy  occupied  the  chair,  in 
1851,  and  the  late  Lord  Wrottesley  nine  years  later,  in  1860, 
other  sciences  have  been  represented  by  the  distinguished 
men  who  have  presided  over  your  meetings.  The  very  re- 
markable discoveries  in  our  knowledge  of  the  heavens 
which  have  taken  place  during  this  period  of  thirty  years — 
one  of  amazing  and  ever-increasing  activity  in  all  branches 
of  science — have  not  passed  unnoticed  in  the  addresses  of 
your  successive  presidents;  still,  it  seems  to  me  fitting  that 
I  should  speak  to  you  to-night  chiefly  of  those  newer 
methods  of  astronomical  research  which  may  have  led  to 
those  discoveries,  and  which  have  become  possible  by  the 
introduction  since  1860  into  the  observatory  of  the  spec- 
troscope and  the  modern  photographic  plate. 

In  1866  I  had  the  honour  of  bringing  before  this  asso- 
ciation, at  one  of  the  evening  lectures,  an  account  of  the 
first  fruits  of  the  novel  and  unexpected  advances  in  our 
knowledge  of  the  celestial  bodies  which  followed  rapidly 
upon  KirchhofFs  original  work  on  the  solar  spectrum  and 
the  interpretation  of  its  lines. 

Since  that  time  a  great  harvest  has  been  gathered  in 
the  same  field  by  many  reapers.  Spectroscopic  astronomy 
has  become  a  distinct  and  acknowledged  branch  of  the  sci- 
ence, possessing  a  large  literature  of  its  own,  and  observa- 
tories specially  devoted  to  it.  The  more  recent  discovery 
of  the  gelatine  dry  plate  has  given  a  further  great  impetus 

1  From  "  Report  of  the  British  Association  for  the  Advancement  of 
Science  "  (1891). 

391 


392  HUGGINS 

to  this  modern  side  of  astronomy,  and  has  opened  a  path- 
way into  the  unknown  of  which  even  an  enthusiast  thirty 
years  ago  would  scarcely  have  dared  to  dream. 

In  no  science,  perhaps,  does  the  sober  statement  of  the 
results  which  have  been  achieved  appeal  so  strongly  to  the 
imagination,  and  make  so  evident  the  almost  boundless 
powers  of  the  mind  of  man.  By  means  of  its  light  alone  to 
analyze  the  chemical  nature  of  a  far-distant  body;  to  be  able 
to  reason  about  its  present  state  in  relation  to  the  past  and 
future;  to  measure  within  an  English  mile  or  less  per  sec- 
ond the  otherwise  invisible  motion  which  it  may  have  to- 
ward us  or  from  us;  to  do  more,  to  make  even  that  which  is 
darkness  to  our  eyes  light,  and  from  vibrations  which  our 
organs  of  sight  are  powerless  to  perceive,  to  evolve  a  reve- 
lation in  which  we  see  mirrored  some  of  the  stages  through 
which  the  stars  may  pass  in  their  slow  evolutional  progress 
— surely  the  record  of  such  achievements,  however  poor  the 
form  of  words  in  which  they  may  be  described,  is  worthy 
to  be  regarded  as  the  scientific  epic  of  the  present  century. 

I  do  not  purpose  to  attempt  a  survey  of  the  progress  of 
spectroscopic  astronomy  from  its  birth  at  Heidelberg  in 
1859,  but  to  point  out  what  we  do  know  at  present,  as  dis- 
tinguished from  what  we  do  not  know,  of  a  few  only  of  its 
more  important  problems,  giving  a  prominent  place,  in  ac- 
cordance with  the  traditions  of  this  chair,  to  the  work  of 
the  last  year  or  two. 

In  the  spectroscope  itself  advances  have  been  made  by 
Lord  Rayleigh  by  his  discussion  of  the  theory  of  the  in- 
strument, and  by  Professor  Rowland  in  the  construction  of 
concave  gratings. 

Lord  Rayleigh  has  shown  that  there  is  not  the  neces- 
sary connection,  sometimes  supposed,  between  dispersion 
and  resolving  power,  as,  besides  the  prism  or  grating,  other 
details  of  construction  and  of  adjustment  of  a  spectroscope 
must  be  taken  into  account. 

The  resolving  power  of  the  prismatic  spectroscope  is 
proportional  to  the  length  of  path  in  the  dispersive  me- 
dium. For  the  heavy  flint  glass  used  in  Lord  Rayleigh's 


CELESTIAL  SPECTROSCOPY  393 

experiments  the  thickness  necessary  to  resolve  the  sodium 
lines  came  out  1.02  centimetre.  If  this  be  taken  as  a  unit, 
the  resolving  power  of  a  prism  of  similar  glass  will  be  in 
the  neighbourhood  of  the  sodium  lines  equal  to  the  number 
of  centimetres  of  its  thickness.  In  other  parts  of  the  spec- 
trum the  resolving  power  will  vary  inversely  as  the  third 
power  of  the  wave-length,  so  that  it  will  be  eight  times  as 
great  in  the  violet  as  in  the  red.  The  resolving  power  of 
a  spectroscope  is  therefore  proportional  to  the  total  thick- 
ness of  the  dispersive  material  in  use,  irrespective  of  the 
number,  the  angles,  or  the  setting  of  the  separate  prisms 
into  which,  for  the  sake  of  convenience,  it  may  be  dis- 
tributed. 

The  resolving  power  of  a  grating  depends  upon  the 
total  number  of  lines  on  its  surface  and  the  order  of  spec- 
trum in  use,  about  one  thousand  lines  being  necessary  to 
resolve  the  sodium  lines  in  the  first  spectrum. 

As  it  is  often  of  importance  in  the  record  of  observations 
to  state  the  efficiency  of  the  spectroscope  with  which  they 
were  made,  Professor  Schuster  has  proposed  the  use  of  a 
unit  of  purity  as  well  as  of  resolving  power,  for  the  full  re- 
solving power  of  a  spectroscope  is  realized  in  practice  only 
when  a  sufficiently  narrow  slit  is  used.  The  unit  of  purity 
also  is  to  stand  for  the  separation  of  two  lines  differing  by 
101ofl  of  their  own  wave-length;  about  the  separation  of  the 
sodium  pair  at  D. 

A  further  limitation  may  come  in  from  the  physiological 
fact  that,  as  Lord  Rayleigh  has  pointed  out,  the  eye  when 
its  full  aperture  is  used  is  not  a  perfect  instrument.  If  we 
wish  to  realize  the  full  resolving  power  of  a  spectroscope, 
therefore,  the  emergent  beam  must  not  be  larger  than  about 
one  third  of  the  opening  of  the  pupil. 

Up  to  the  present  time  the  standard  of  reference  for 
nearly  all  spectroscopic  work  continues  to  be  Angstrom's 
map  of  the  solar  spectrum,  and  his  scale  based  upon  his 
original  determinations  of  absolute  wave-length.  It  is  well 
known,  as  was  pointed  out  by  Thalen  in  his  work  on  the 
spectrum  of  iron  in  1884,  that  Angstrom's  figures  are 


394 


HUGGINS 


slightly  too  small,  in  consequence  of  an  error  existing  in 
a  standard  metre  used  by  him.  The  corrections  for  this 
have  been  introduced  into  the  tables  of  the  wave-lengths 
of  terrestrial  spectra  collected  and  revised  by  a  commit- 
tee of  this  association  from  1885  to  1887.  Last  year  the 
committee  added  a  table  of  corrections  to  Rowland's 
scale. 

The  inconvenience  caused  by  a  change  of  standard 
scale  is,  for  a  time  at  least,  considerable;  but  there  is  little 
doubt  that  in  the  near  future  Rowland's  photographic  map 
of  the  solar  spectrum,  and  his  scale  based  on  the  determina- 
tions of  absolute  wave-length  by  Peirce  and  Bell,  or  the 
Potsdam  scale  based  on  original  determinations  oy  Miiller 
and  Kempf,  which  differs  very  slightly  from  it,  will  come 
to  be  exclusively  adopted. 

The  great  accuracy  of  Rowland's  photographic  map  is 
due  chiefly  to  the  introduction  by  him  of  concave  gratings, 
and  of  a  method  for  their  use,  by  which  the  problem  of  the 
determination  of  relative  wave-lengths  is  simplified  to  meas- 
ures of  near  coincidences  of  the  lines  in  different  spectra  by 
a  micrometer. 

The  concave  grating  and  its  peculiar  mounting,  in 
which  no  lenses  or  telescope  are  needed,  and  in  which  all 
the  spectra  are  in  focus  together,  formed  a  new  departure  of 
great  importance  in  the  measurement  of  spectral  lines.  The 
valuable  method  of  photographic  sensitizers  for  different 
parts  of  the  spectrum  has  enabled  Professor  Rowland  to 
include  in  his  map  the  whole  visible  solar  spectrum,  as  well 
as  the  ultra-violet  portion  as  far  as  it  can  get  through  our 
atmosphere.  Some  recent  photographs  of  the  solar  spec- 
trum, which  include  A,  by  Mr.  George  Higgs,  are  of  great 
technical  beauty. 

During  the  past  year  the  results  of  three  independent 
researches  have  appeared,  in  which  the  special  object  of 
the  observers  has  been  to  distinguish  the  lines  which  are 
due  to  our  atmosphere  from  those  which  are  truly  solar — 
the  maps  of  M.  Thollon,  which,  owing  to  his  lamented 
death  just  before  their  final  completion,  have  assumed  the 


CELESTIAL  SPECTROSCOPY  395 

character  of  a  memorial  to  him;  maps  by  Mr.  Becker;  and 
sets  of  photographs  of  a  high  and  a  low  sun  by  Mr.  Mc- 
Clean. 

At  the  meeting  of  this  association  in  Bath.  Mr.  Janssen 
gave  an  account  of  his  own  researches  on  the  terrestrial 
lines  of  the  solar  spectrum,  which  owe  their  origin  to  the 
oxygen  of  our  atmosphere.  He  discovered  the  remarkable 
fact  that  while  the  intensity  of  one  class  of  bands  varies  as 
the  density  of  the  gas,  other  diffuse  bands  vary  as  the 
square  of  the  density.  These  observations  are  in  accord- 
ance with  the  work  of  Egoroff  and  of  Olszewski,  and  of 
Liveing  and  Dewar  on  condensed  oxygen.  In  some  recent 
experiments  Olszewski,  with  a  layer  of  liquid  oxygen  thirty 
millimetres  thick,  saw,  as  well  as  four  other  bands,  the  band 
coincident  with  Fraunhofer's  A;  a  remarkable  instance  of 
the  persistence  of  absorption  through  a  great  range  of 
temperature.  The  light  which  passed  through  the  liquid 
oxygen  had  a  light-blue  colour  resembling  that  of  the  sky. 

Of  not  less  interest  are  the  experiments  of  Angstrom 
which  show  that  the  carbonic  acid  and  aqueous  vapour 
of  the  atmosphere  reveal  their  presence  by  dark  bands  in 
the  invisible  infra-red  region,  at  the  positions  of  bands  of 
emission  of  these  substances. 

It  is  now  some  thirty  years  since  the  spectroscope  gave 
us  for  the  first  time  certain  knowledge  of  the  nature  of  the 
heavenly  bodies,  and  revealed  the  fundamental  fact  that  ter- 
restrial matter  is  not  peculiar  to  the  solar  system,  but  that 
it  is  common  to  all  the  stars  which  are  visible  to  us. 

In  the  case  of  a  star,  such  as  Capella,  which  has  a 
spectrum  almost  identical  with  that  of  the  sun,  we  feel  justi- 
fied in  concluding  that  the  matter  of  which  it  is  built  up 
is  similar,  and  that  its  temperature  is  also  high,  and  not  very 
different  from  the  solar  temperature.  The  task  of  analyzing 
the  stars  and  nebulae  becomes,  however,  one  of  very  great 
difficulty  when  we  have  to  do  with  spectra  differing  from 
the  solar  type.  We  are  thrown  back  upon  the  laboratory 
for  the  information  necessary  to  enable  us  to  interpret  the 
indications  of  the  spectroscope  as  to  the  chemical  nature, 


396  HUGGINS 

the  density  and  pressure,  and  the  temperature  of  the  celes- 
tial masses. 

What  the  spectroscope  immediately  reveals  to  us  are 
the  waves  which  were  set  up  in  the  ether  filling  all  inter- 
stellar space,  years  or  hundreds  of  years  ago,  by  the  motion 
of  the  molecules  of  the  celestial  substances.  As  a  rule  it 
is  only  when  a  body  is  gaseous  and  sufficiently  hot  that  the 
motions  within  its  molecules  can  produce  bright  lines  and 
a  corresponding  absorption.  The  spectra  of  the  heavenly 
bodies  are,  indeed,  to  a  great  extent  absorption  spectra,  but 
we  have  naturally  to  study  them  through  the  corresponding 
emission  spectra  of  bodies  brought  into  the  gaseous  form 
and  rendered  luminous  by  means  of  flames  or  of  electric 
discharges.  In  both  cases,  unfortunately,  it  has  been  shown 
recently  by  Professors  Liveing  and  Dewar,  Wullner,  E. 
Wiedemann,  and  others,  that  there  appears  to  be  no  certain 
direct  relation  between  the  luminous  radiation  as  shown  in 
the  spectroscope  and  the  temperature  of  the  flame,  or  of 
the  gaseous  contents  of  the  vacuum  tube — that  is,  in  the 
usual  sense  of  the  term  as  applied  to  the  mean  motion  of 
all  the  molecules.  In  both  cases  the  vibratory  motions 
within  the  molecules  to  which  their  luminosity  is  due  are 
almost  always  much  greater  than  would  be  produced  by 
encounters  of  molecules  having  motions  of  translation  no 
greater  than  the  average  motions  which  characterize  the 
temperature  of  the  gases  as  a  whole.  The  temperature  of 
a  vacuum  tube  through  which  an  electric  discharge  is  tak- 
ing place  may  be  low,  as  shown  thermometrically,  quite 
apart  from  the  consideration  of  the  extreme  smallness  of 
the  mass  of  gas,  but  the  vibrations  of  the  luminous  mole- 
cules must  be  violent  in  whatever  way  we  suppose  them  to 
be  set  up  by  the  discharge;  if  we  take  Schuster's  view  that 
comparatively  few  molecules  are  carrying  the  discharge, 
and  that  it  is  to  the  fierce  encounters  of  these  alone  that 
the  luminosity  is  due,  then  if  all  the  molecules  had  simi- 
lar motions,  the  temperature  of  the  gas  would  be  very 
high. 

So  in  flames  where  chemical  changes  are  in  progress, 


CELESTIAL  SPECTROSCOPY  397 

the  vibratory  motions  of  the  molecules  which  are  luminous 
may  be,  in  connection  with  the  energy  set  free  in  these 
changes,  very  different  from  those  corresponding  to  the 
mean  temperature  of  the  flame.  Under  the  ordinary  con- 
ditions of  terrestrial  experiments,  therefore,  the  tempera- 
ture or  the  mean  vis  viva  of  the  molecules  may  have  no 
direct  relation  to  the  total  radiation,  which,  on  the  other 
hand,  is  the  sum  of  the  radiation  due  to  each  luminous 
molecule.  These  phenomena  have  recently  been  discussed 
by  Ebert  from  the  standpoint  of  the  electro-magnetic  theory 
of  light. 

Very  great  caution  is  therefore  called  for  when  we  at- 
tempt to  reason  by  the  aid  of  laboratory  experiments  to 
the  temperature  of  the  heavenly  bodies  from  their  radia- 
tion, especially  on  the  reasonable  assumption  that  in  them 
the  luminosity  is  not  ordinarily  associated  with  chemical 
changes  or  with  electrical  discharges,  but  is  due  to  a  simple 
glowing  from  the  ultimate  conversion  into  molecular  mo- 
tion of  the  gravitational  energy  of  shrinkage. 

In  a  recent  paper  Stas  maintains  that  electric  spectra  are 
to  be  regarded  as  distinct  from  flame  spectra,  and,  from  re- 
searches of  his  own,  that  the  pairs  of  lines  of  the  sodium 
spectrum  other  than  D  are  produced  only  by  disruptive 
electric  discharges.  As  these  pairs  of  lines  are  found  re- 
versed in  the  solar  spectrum,  he  concludes  that  the  sun's 
radiation  is  due  mainly  to  electric  discharges.  But  Wolf 
and  Diacon,  and  later  Watts,  observed  the  other  pairs  of 
lines  of  the  sodium  spectrum  when  the  vapour  was  raised 
above  the  ordinary  temperature  of  the  Bunsen  flame.  Re- 
cently Liveing  and  Dewar  saw  easily,  besides  D,  the  citron 
and  green  pairs,  and  sometimes  the  blue  pair  and  the  orange 
pair,  when  hydrogen  charged  with  sodium  vapour  was  burn- 
ing at  different  pressures  in  oxygen.  In  the  case  of  sodium 
vapours,  therefore,  and  presumably  in  all  other  vapours  and 
gases,  it  is  a  matter  of  indifference  whether  the  necessary 
vibratory  motion  of  the  molecules  is  produced  by  electric 
discharges  or  by  flames.  The  presence  of  lines  in  the  solar 
spectrum  which  we  can  only  produce  electrically  is  an  indi- 


398  HUGGINS 

cation,  however,  as  Stas  points  out,  of  the  high  tempera- 
ture of  the  sun. 

We  must  not  forget  that  the  light  from  the  heavenly 
bodies  may  consist  of  the  combined  radiations  of  different 
layers  of  gas  at  different  temperatures,  and  possibly  be  fur- 
ther complicated  to  an  unknown  extent  by  the  absorption 
of  cooler  portions  of  gas  outside. 

Not  less  caution  is  needed  if  we  endeavour  to  argue 
from  the  broadening  of  lines  and  the  coming  in  of  a  con- 
tinuous spectrum  as  to  the  relative  pressure  of  the  gas  in 
the  celestial  atmospheres.  On  the  one  hand,  it  can  not  be 
gainsaid  that  in  the  laboratory  the  widening  of  the  lines 
in  a  Plucker's  tube  follows  upon  increasing  the  density  of 
the  residue  of  hydrogen  in  the  tube,  when  the  vibrations 
are  more  frequently  disturbed  by  fresh  encounters;  and  that 
a  broadening  of  the  sodium  lines  in  a  flame  at  ordinary 
pressure  is  produced  by  an  increase  of  the  quantity  of 
sodium  in  the  flame;  but  it  is  doubtful  if  pressure,  as  dis- 
tinguished from  quantity,  does  produce  an  increase  of  the 
breadth  of  the  lines.  An  individual  molecule  of  sodium  will 
be  sensibly  in  the  same  condition,  considering  the  relatively 
enormous  number  of  the  molecules  of  the  other  gases, 
whether  the  flame  is  scantily  or  copiously  fed  with  the 
sodium  salt.  With  a  small  quantity  of  sodium  vapour  the 
intensity  will  be  feeble  except  near  the  maximum  of  the 
lines;  when,  however,  the  quantity  is  increased,  the  com- 
parative transparency  on  the  sides  of  the  maximum  will 
allow  the  light  from  the  additional  molecules  met  with  in 
the  path  of  the  visual  ray  to  strengthen  the  radiation  of 
the  molecules  farther  back,  and  so  increase  the  breadth  of 
the  lines. 

In  a  gaseous  mixture  it  is  found,  as  a  rule,  that  at  the 
same  pressure  or  temperature,  as  the  encounters  with  simi- 
lar molecules  become  fewer,  the  spectral  lines  will  be 
affected  as  if  the  body  were  observed  under  conditions  of 
reduced  quantity  or  temperature. 

In  their  recent  investigation  of  the  spectroscopic  behav- 
iour of  flames  under  various  pressures  up  to  forty  atmos- 


CELESTIAL  SPECTROSCOPY  399 

pheres,  Professors  Liveing  and  Dewar  have  come  to  the 
conclusion  that  though  the  prominent  feature  of  the  light 
emitted  by  flames  at  high  pressure  appears  to  be  a  strong 
continuous  spectrum,  there  is  not  the  slightest  indication 
that  this  continuous  spectrum  is  produced  by  the  broad- 
ening of  the  lines  of  the  same  gases  at  low  pressure.  On 
the  contrary,  photometric  observations  of  the  brightness 
of  the  continuous  spectrum,  as  the  pressure  is  varied,  show 
that  it  is  mainly  produced  by  the  mutual  action  of  the 
molecules  of  a  gas.  Experiments  on  the  sodium  spec- 
trum were  carried  up  to  a  pressure  of  forty  atmospheres 
without  producing  any  definite  effect  on  the  width  of 
the  lines  which  could  be  ascribed  to  the  pressure.  In  a 
similar  way  the  lines  of  the  spectrum  of  water  showed  no 
signs  of  expansion  up  to  twelve  atmospheres;  though  more 
intense  than  at  ordinary  pressure,  they  remained  narrow 
and  clearly  defined. 

It  follows,  therefore,  that  a  continuous  spectrum  can 
not  be  considered,  when  taken  alone,  as  a  sure  indication 
of  matter  in  the  liquid  or  the  solid  state.  Not  only,  as  in 
the  experiments  already  mentioned,  such  a  spectrum  may 
be  due  to  gas  when  under  pressure,  but,  as  Maxwell  pointed 
out,  if  the  thickness  of  a  medium,  such  as  sodium  vapour, 
which  radiates  and  absorbs  different  kinds  of.  light,  be  very 
great,  and  the  temperature  high,  the  light  emitted  will  be 
of  exactly  the  same  composition  as  that  emitted  by  lamp- 
black at  the  same  temperature,  for  the  radiations  which  are 
feebly  emitted  will  be  also  feebly  absorbed,  and  can  reach 
the  surface  from  immense  depths.  Schuster  has  shown 
that  oxygen,  even  in  a  partially  exhausted  tube,  can  give  a 
continuous  spectrum  when  excited  by  a  feeble  electric  dis- 
charge. 

Compound  bodies  are  usually  distinguished  by  a  banded 
spectrum;  but  on  the  other  hand  such  a  spectrum  does  not 
necessarily  show  the  presence  of  compounds — that  is,  of 
molecules  containing  different  kinds  of  atoms — but  simply 
of  a  more  complex  molecule,  wrhich  may  be  made  up  of 
similar  atoms,  and  be  therefore  an  allotropic  condition  of 


400  HUGGINS 

the  same  body.  In  some  cases,  for  example,  in  the  diffuse 
bands  of  the  absorption  spectrum  of  oxygen,  the  bands 
may  have  an  intensity  proportional  to  the  square  of  the 
density  of  the  gas,  and  may  be  due  either  to  the  formation 
of  more  complex  molecules  of  the  gas  with  increase  of  pres- 
sure, or  it  may  be  to  the  constraint  to  which  the  molecules 
are  subject  during  their  encounters  with  one  another. 

It  may  be  thought  that  at  least  in  the  coincidences  of 
bright  lines  we  are  on  the  solid  ground  of  certainty,  since 
the  length  of  the  waves  set  up  in  the  ether  by  a  molecule, 
say  of  hydrogen,  is  the  most  fixed  and  absolutely  perma- 
nent quantity  in  Nature,  and  is  so  of  physical  necessity, 
for  with  any  alteration  the  molecule  would  cease  to  be 
hydrogen. 

Such  would  be  the  case  if  the  coincidence  were  certain; 
but  an  absolute  coincidence  can  be  only  a  matter  of  greater 
or  less  probability,  depending  on  the  resolving  power  em- 
ployed, on  the  number  of  the  lines  which  correspond  and 
on  their  characters.  When  the  coincidences  are  very  nu- 
merous, as  in  the  case  of  iron  and  the  solar  spectrum,  or 
the  lines  are  characteristically  grouped,  as  in  the  case  of 
hydrogen  and  the  solar  spectrum,  we  may  regard  the  coin- 
cidence as  certain;  but  the  progress  of  science  has  been 
greatly  retarded  by  resting  important  conclusions  upon  the 
apparent  coincidence  of  single  lines,  in  spectroscopes  of 
very  small  resolving  power.  In  such  cases,  unless  other 
reasons  supporting  the  coincidence  are  present,  the  proba- 
bility of  a  real  coincidence  is  almost  too  small  to  be  of  any 
importance,  especially  in  the  case  of  a  heavenly  body  which 
may  have  a  motion  of  approach  or  of  recession  of  unknown 
amount. 

But  even  here  we  are  met  by  the  confusion  introduced 
by  multiple  spectra,  corresponding  to  different  molecular 
groupings  of  the  same  substance;  and  further,  to  the  in- 
fluence of  substances  in  vapour  upon  each  other;  for  when 
several  gases  are  present  together,  the  phenomena  of  radia- 
tion and  reversal  by  absorption  are  by  no  means  the  same 
as  if  the  gases  were  free  from  each  other's  influence,  and 


CELESTIAL   SPECTROSCOPY  4OI 

especially  is  this  the  case  when  they  are  illuminated  by  an 
electric  discharge. 

I  have  said  as  much  as  time  will  permit,  and  I  think 
indeed  sufficient,  to  show  that  it  is  only  by  the  laborious 
and  slow  process  of  most  cautious  observations  that  the 
foundations  of  the  science  of  celestial  physics  can  be  surely 
laid.  We  are  at  present  in  a  time  of  transition  when  the 
earlier  and,  in  the  nature  of  things,  less  precise  observa- 
tions are  giving  place  to  work  of  an  order  of  accuracy  much 
greater  than  was  formerly  considered  attainable  with  ob- 
jects of  such  small  brightness  as  the  stars. 

The  accuracy  of  the  earlier  determinations  of  the  spectra 
of  the  terrestrial  elements  is  in  most  cases  insufficient  for 
modern  work  on  the  stars  as  well  as  on  the  sun.  They  fall 
much  below  the  scale  adopted  in  Rowland's  map  of  the 
sun,  as  well  as  below  the  degree  of  accuracy  attained  at 
Potsdam  by  photography  in  a  part  of  the  spectrum  for 
the  brighter  stars.  Increase  of  resolving  power  very  fre- 
quently breaks  up  into  groups,  in  the  spectra  of  the  sun 
and  stars,  the  lines  which  had  been  regarded  as  single,  and 
their  supposed  coincidences  with  terrestrial  lines  fall  to 
the  ground.  For  this  reason  many  of  the  early  conclusions, 
based  on  observations  as  good  as  it  was  possible  to  make 
at  the  time  with  the  less  powerful  spectroscopes  then  in  use, 
may  not  be  found  to  be  maintained  under  the  much  greater 
resolving  power  of  modern  instruments. 

The  spectroscope  has  failed  as  yet  to  interpret  for  us  the 
remarkable  spectrum  of  the  aurora  borealis.  Undoubtedly 
in  this  phenomenon  portions  of  our  atmosphere  are  lighted 
up  by  electric  discharges;  we  should  expect,  therefore,  to 
recognise  the  spectra  of  the  gases  known  to  be  present  in 
it.  As  yet  we  have  not  been  able  to  obtain  similar  spectra 
from  these  gases  artificially,  and  especially  wre  do  not  know 
the  origin  of  the  principal  line  in  the  green,  which  often 
appears  alone,  and  may  have  therefore  an  origin  independ- 
ent of  that  of  the  other  lines.  Recently  the  suggestion  has 
been  made  that  the  aurora  is  a  phenomenon  produced  by 
the  dust  of  meteors  and  falling  stars,  and  that  near  positions 
26 


402  HUGGINS 

of  certain  auroral  lines  to  lines  or  flutings  of  manganese, 
lead,  barium,  thallium,  iron,  etc.,  are  sufficient  to  justify  us 
in  regarding  meteoric  dust  in  the  atmosphere  as  the  origin 
of  the  auroral  spectrum.  Liveing  and  Dewar  have  made  a 
conclusive  research  on  this  point  by  availing  themselves  of 
the  dust  of  excessive  minuteness  thrown  off  from  the  sur- 
face of  electrodes  of  various  metals  and  meteorites  by  a 
disruptive  discharge  and  carried  forward  into  the  tube  of 
observation  by  a  more  or  less  rapid  current  of  air  or  other 
gas.  These  experiments  prove  that  metallic  dust,  however 
fine,  suspended  in  a  gas  will  not  act  like  gaseous  matter  in 
becoming  luminous  with  its  characteristic  spectrum  in  an 
electric  discharge,  similar  to  that  of  the  aurora.  Professor 
Schuster  has  suggested  that  the  principal  line  may  be  due 
to  some  very  light  gas  which  is  present  in  too  small  a  pro- 
portion to  be  detected  by  chemical  analysis  or  even  by  the 
spectroscope  in  the  presence  of  the  other  gases  near  the 
earth,  but  which  at  the  height  of  the  auroral  discharges  is 
in  a  sufficiently  greater  relative  proportion  to  give  a  spec- 
trum. Lemstrom,  indeed,  states  that  he  saw  this  line  in  the 
silent  discharge  of  a  Holtz  machine  on  a  mountain  in  Lap- 
land. The  lines  may  not  have  been  obtained  in  our  labora- 
tories from  the  atmospheric  gases,  on  account  of  the  diffi- 
culty of  reproducing  in  tubes  with  sufficient  nearness  the 
conditions  under  which  the  auroral  discharges  take  place. 
In  the  spectra  of  comets  the  spectroscope  has  shown  the 
presence  of  carbon  presumably  in  combination  with  hydro- 
gen, and  also  sometimes  with  nitrogen;  and  in  the  case  of 
comets  approaching  very  near  the  sun,  the  lines  of  sodium, 
and  other  lines  which  have  been  supposed  to  belong  to  iron. 
Though  the  researches  of  Professor  H.  A.  Newton  and  of 
Professor  Schiaparelli  leave  no  doubt  of  the  close  connec- 
tion of  comets  with  corresponding  periodic  meteor  swarms, 
and  therefore  of  the  probable  identity  of  cometary  matter 
with  that  of  meteorites,  with  which  the  spectroscopic  evi- 
dence agrees,  it  would  be  perhaps  unwise  at  present  to 
attempt  to  define  too  precisely  the  exact  condition  of  the 
matter  which  forms  the  nucleus  of  the  comet.  In  any 


CELESTIAL  SPECTROSCOPY  403 

case  the  part  of  the  light  of  the  comet  which  is  not  reflected 
solar  light  can  scarcely  be  attributed  to  a  high  tempera- 
ture produced  by  the  clashing  of  separate  meteoric  stones 
set  up  within  the  nucleus  by  the  sun's  disturbing  force. 
We  must  look  rather  to  disruptive  electric  discharges  pro- 
duced probably  by  processes  of  evaporation  due  to  increased 
solar  heat,  which  would  be  amply  sufficient  to  set  free  por- 
tions of  the  occluded  gases  into  the  vacuum  of  space.  May 
it  be  that  these  discharges  are  assisted,  and  indeed  pos- 
sibly increased,  by  the  recently  discovered  action  of  the 
ultra-violet  part  of  the  sun's  light?  Hertz  has  shown  that 
ultra-violet  light  can  produce  a  discharge  from  a  negatively 
electrified  piece  of  metal,  while  Hallwachs  and  Righi  have 
shown  further  that  ultra-violet  light  can  even  charge  posi- 
tively an  unelectrified  piece  of  metal — phenomena  which 
Lenard  and  Wolf  associate  with  the  disengagement  from 
the  metallic  surfaces  of  very  minute  particles.  Similar 
actions  on  cometary  matter,  unscreened  as  it  is  by  an  ab- 
sorptive atmosphere,  at  least  of  any  noticeable  extent,  may 
well  be  powerful  when  a  comet  approaches  the  sun,  and 
help  to  explain  an  electrified  condition  of  the  evaporated 
matter  which  would  possibly  bring  it  under  the  sun's  re- 
pulsive action.  We  shall  have  to  return  to  this  point  in 
speaking  of  the  solar  corona. 

A  very  great  advance  has  been  made  in  our  knowledge 
of  the  constitution  of  the  sun  by  the  recent  work  at  the 
Johns  Hopkins  University  by  means  of  photography  and 
concave  gratings,  in  comparing  the  solar  spectrum,  under 
the  great  resolving  power,  directly  with  the  spectra  of  the 
terrestrial  elements.  Professor  Rowland  has  shown  that 
the  lines  of  thirty-six  terrestrial  elements  at  least  are  cer- 
tainly present  in  the  solar  spectrum,  while  eight  others  are 
doubtful.  Fifteen  elements,  including  nitrogen  as  it  shows 
itself  under  an  electric  discharge  in  a  vacuum  tube,  have 
not  been  found  in  the  solar  spectrum.  Some  ten  other  ele- 
ments, inclusive  of  oxygen,  have  not  yet  been  compared 
with  the  sun's  spectrum. 

Rowland  remarks  that  of  the  fifteen  elements  named  as 


404  HUGGINS 

not  found  in  the  sun,  many  are  so  classed  because  they  have 
few  strong  lines,  or  none  at  all,  in  the  limit  of  the  solar  spec- 
trum as  compared  by  him  with  the  arc.  Boron  has  only 
two  strong  lines.  The  lines  of  bismuth  are  compound  and 
too  diffuse.  Therefore,  even  in  the  case  of  these  fifteen  ele- 
ments, there  is  little  evidence  that  they  are  really  absent 
from  the  sun. 

It  follows  that  if  the  whole  earth  were  heated  to  the 
temperature  of  the  sun,  its  spectrum  would  resemble  very 
closely  the  solar  spectrum. 

Rowland  has  not  found  any  lines  common  to  several 
elements,  and  in  the  case  of  some  accidental  coincidences 
more  accurate  investigation  reveals  some  slight  difference 
of  wave-length  or  a  common  impurity.  Further,  the  rela- 
tive strength  of  the  lines  in  the  solar  spectrum  is  generally, 
with  a  few  exceptions,  the  same  as  that  in  the  electric  arc, 
so  that  Rowland  considers  that  his  experiments  show 
"  very  little  evidence  "  of  the  breaking  up  of  the  terrestrial 
elements  in  the  sun. 

Stas,  in  a  recent  paper,  gives  the  final  results  of  eleven 
years  of  research  on  the  chemical  elements  in  a  state  of 
purity,  and  on  the  possibility  of  decomposing  them  by  the 
physical  and  chemical  forces  at  our  disposal.  His  experi- 
ments on  calcium,  strontium,  lithium,  magnesium,  silver, 
sodium,  and  thallium  show  that  these  substances  retain  their 
individuality  under  all  conditions,  and  are  unalterable  by 
any  forces  that  we  can  bring  to  bear  upon  them. 

Professor  Rowland  looks  to  the  solar  lines  which  are 
unaccounted  for  as  a  means  of  enabling  him  to  discover 
such  new  terrestrial  elements  as  still  lurk  in  raw  minerals 
and  earths,  by  confronting  their  spectra  directly  with  that 
of  the  sun.  He  has  already  resolved  yttrium  spectro- 
scopically  into  three  compounds,  and  actually  into  two. 
The  comparison  of  the  results  of  this  independent  analyt- 
ical method  with  the  remarkable  but  different  conclusions 
to  which  M.  Lecoq  de  Boisbaudran  and  Mr.  Crookes  have 
been  led  respectively,  from  spectroscopic  observation  on 
these  bodies  when  glowing  under  molecular  bombardment 


CELESTIAL  SPECTROSCOPY  405 

in  a  vacuum  tube,  will  be  awaited  with  much  interest.  It 
is  worthy  of  remark  that  as  our  knowledge  of  the  spectrum 
of  hydrogen  in  its  complete  form  came  to  us  from  the  stars, 
it  is  now  from  the  sun  that  chemistry  is  probably  about  to 
be  enriched  by  the  discovery  of  new  elements. 

In  a  discussion  in  the  Bakerian  lecture  for  1885  °f  what 
we  knew  up  to  that  time  of  the  sun's  corona,  I  was  led  to 
the  conclusion  that  the  corona  is  essentially  a  phenomenon 
similar  in  the  cause  of  its  formation  to  the  tails  of  comets — 
namely,  that  it  consists  for  the  most  part  probably  of  mat- 
ter going  from  the  sun  under  the  action  of  a  force,  possibly 
electrical,  which  varies  as  the  surface,  and  can  therefore  in 
the  case  of  highly  attenuated  matter  easily  master  the  force 
of  gravity  even  near  the  sun.  Though  many  of  the  coronal 
particles  may  return  to  the  sun,  those  which  form  the  long 
rays  or  streamers  do  not  return;  they  separate  and  soon 
become  too  diffused  to  be  any  longer  visible,  and  may  well 
go  to  furnish  the  matter  of  the  zodiacal  light,  which  other- 
wise has  not  received  a  satisfactory  explanation.  And  fur- 
ther, if  such  a  force  exist  at  the  sun,  the  changes  of  terres- 
trial magnetism  may  be  due  to  direct  electric  action,  as  the 
earth  moves  through  lines  of  inductive  force. 

These  conclusions  seem  to  be  in  accordance  broadly 
with  the  lines  along  which  thought  has  been  directed  by 
the  results  of  subsequent  eclipses.  Professor  Schuster  takes 
an  essentially  similar  view,  and  suggests  that  there  may  be 
a  direct  electric  connection  between  the  sun  and  the  planets. 
He  asks  further  whether  the  sun  may  not  act  like  a  magnet 
in  consequence  of  its  revolution  about  its  axis.  Professor 
Bigelow  has  recently  treated  the  coronal  forms  by  the 
theory  of  spherical  harmonics,  on  the  supposition  that  we 
see  phenomena  similar  to  those  of  free  electricity,  the  rays 
being  lines  of  force,  and  the  coronal  matter  discharged 
from  the  sun,  or  at  least  arranged  or  controlled  by  these 
forces.  At  the  extremities  of  the  streams  for  some  reasons 
the  repulsive  power  may  be  lost,  and  gravitation  set  in, 
bringing  the  matter  back  to  the  sun.  The  matter  which 
does  leave  the  sun  is  persistently  transported  to  the  equa- 


406  HUGGINS 

torial  plane  of  the  corona;  in  fact,  the  zodiacal  light  may 
be  the  accumulation  at  great  distances  from  the  sun  along 
this  equator  of  similar  material.  Photographs  on  a  larger 
scale  will  be  desirable  for  the  full  development  of  the  con- 
clusions which  may  follow  from  this  study  of  the  curved 
forms  of  the  coronal  structure.  Professor  Schaeberle,  how- 
ever, considers  that  the  coronal  phenomena  may  be  satis- 
factorily accounted  for  on  the  supposition  that  the  corona 
is  formed  of  streams  of  matter  ejected  mainly  from  the 
spot  zones  with  great  initial  velocities,  but  smaller  than 
382  miles  a  second;  further,  that  the  different  types  of 
the  corona  are  due  to  the  effects  of  perspective  on  the 
streams  from  the  earth's  place  at  the  time  relatively  to  the 
plane  of  the  solar  equator. 

Of  the  physical  and  the  chemical  nature  of  the  coronal 
matter  we  know  very  little.  Schuster  concludes,  from  an 
examination  of  the  eclipses  of  1882,  1883,  and  1886,  that 
the  continuous  spectrum  of  the  corona  has  the  maximum 
of  actinic  intensity  displaced  considerably  toward  the  red 
when  compared  with  the  spectrum  of  the  sun,  which  shows 
that  it  can  only  be  due  in  small  part  to  solar  light  scattered 
by  small  particles.  The  lines  of  calcium  and  of  hydrogen 
do  not  appear  to  form  part  of  the  normal  spectrum  of  the 
corona.  The  green  coronal  line  has  no  known  representa- 
tive in  terrestrial  substances,  nor  has  Schuster  been  able 
to  recognise  any  of  our  elements  in  the  other  lines  of  the 
corona. 

The  spectra  of  the  stars  are  almost  infinitely  diversified, 
yet  they  can  be  arranged,  with  some  exceptions,  in  a  series 
in  which  the  adjacent  spectra,  especially  in  the  photographic 
region,  are  scarcely  distinguishable,  passing  from  the  bluish- 
white  stars  like  Sirius,  through  stars  more  or  less  solar 
in  character,  to  stars  with  banded  spectra,  which  divide 
themselves  into  two  apparently  independent  groups,  ac- 
cording as  the  stronger  edge  of  the  bands  is  toward  the 
red  or  the  blue.  In  such  an  arrangement  the  sun's  place 
is  toward  the  middle  of  the  series. 

At  present  a  difference  of  opinion  exists  as  to  the  direc- 


CELESTIAL  SPECTROSCOPY  407 

tion  in  the  series  in  which  evolution  is  proceeding:  whether 
by  further  condensation  white  stars  pass  into  the  orange 
and  red  stages,  or  whether  these  more  coloured  stars  are 
younger  and  will  become  white  by  increasing  age.  The 
latter  view  was  suggested  by  Johnstone  Stoney  in  1867. 

About  ten  years  ago  Ritter,  in  a  series  of  papers,  dis- 
cussed the  behaviour  of  gaseous  masses  during  condensa- 
tion, and  the  probable  resulting  constitution  of  the  heav- 
enly bodies.  According  to  him,  a  star  passes  through  the 
orange  and  red  stages  twice,  first  during  a  comparatively 
short  period  of  increasing  temperature  which  culminates  in 
the  white  stage,  and  a  second  time  during  a  more  prolonged 
stage  of  gradual  cooling.  He  suggested  that  the  two 
groups  of  banded  stars  may  correspond  to  those  different 
periods,  the  young  stars  being  those  in  which  the  stronger 
edge  of  the  dark  band  is  toward  the  blue,  the  other  banded 
stars,  which  are  relatively  less  luminous  and  few  in  number, 
being  those  which  are  approaching  extinction  through  age. 

Recently  a  similar  evolutional  order  has  been  sug- 
gested, which  is  based  upon  the  hypothesis  that  the  nebulae 
and  stars  consist  of  colliding  meteoric  stones  in  different 
stages  of  condensation. 

More  recently  the  view  has  been  put  forward  that  the 
diversified  spectra  of  the  stars  do  not  represent  the  stages 
of  an  evolutional  progress,  but  are  due  for  the  most  part 
to  differences  of  original  constitution. 

The  few  minutes  which  can  be  given  to  this  part  of  the 
address  are  insufficient  for  a  discussion  of  these  different 
views.  I  purpose,  therefore,  to  state  briefly,  and  with  re- 
serve, as  the  subject  is  obscure,  some  of  the  considerations 
from  the  characters  of  their  spectra  which  appeared  to  me 
to  be  in  favour  of  the  evolutional  order  in  which  T  arranged 
the  stars  from  their  photographic  spectra  in  1879.  This 
order  is  essentially  the  same  as  Vogel  had  previously  pro- 
posed in  his  classification  of  the  stars  in  1874,  in  which  the 
white  stars,  which  are  most  numerous,  represent  the  early 
adult  and  most  persistent  stage  of  stellar  life,  the  solar  con- 
dition that  of  full  maturity  and  of  commencing  age;  while 


408  HUGGINS 

in  the  orange  and  red  stars  with  banded  spectra  we  see  the 
setting  in  and  advance  of  old  age.  But  this  statement  must 
be  taken  broadly,  and  not  as  asserting  that  all  stars,  how- 
ever different  in  mass  and  possibly  to  some  small  extent  in 
original  constitution,  exhibit  one  invariable  succession  of 
spectra. 

In  the  spectra  of  the  white  stars  the  dark  metallic  lines 
are  relatively  inconspicuous  and  occasionally  absent,  at  the 
same  time  that  the  dark  lines  of  hydrogen  are  usually  strong, 
and  more  or  less  broad,  upon  a  continuous  spectrum,  which 
is  remarkable  for  its  brilliancy  at  the  blue  end.  In  some  of 
these  stars  the  hydrogen  and  some  other  lines  are  bright 
and  sometimes  variable. 

As  the  greater  or  less  prominence  of  the  hydrogen  lines, 
dark  or  bright,  is  characteristic  of  the  white  stars  as  a  class, 
and  diminishes  gradually  with  the  incoming  and  increase 
in  strength  of  the  other  lines,  we  are  probably  justified  in 
regarding  it  as  due  to  some  conditions  which  occur  natu- 
rally during  the  progress  of  stellar  life  and  not  to  a  pecul- 
iarity of  original  constitution. 

To  produce  a  strong  absorption-spectrum  a  substance 
must  be  at  the  particular  temperature  at  which  it  is  notably 
absorptive;  and,  further,  this  temperature  must  be  suf- 
ficiently below  that  of  the  region  behind  from  which  the 
light  comes  for  the  gas  to  appear,  so  far  as  its  special  rays 
are  concerned,  as  darkness  upon  it.  Considering  the  high 
temperature  to  which  hydrogen  must  be  raised  before  it 
can  show  its  characteristic  emission  and  absorption,  we 
shall  probably  be  right  in  attributing  the  relative  feebleness 
or  absence  of  the  other  lines,  not  to  the  paucity  of  the 
metallic  vapours,  but  rather  to  their  being  so  hot  relatively 
to  the  substance  behind  them  as  to  show  feebly,  if  at  all,  by 
reversion.  Such  a  state  of  things  would  more  probably  be 
found,  it  seems  to  me,  in  conditions  anterior  to  the  solar 
stage.  A  considerable  cooling  of  the  sun  would  probably 
give  rise  to  banded  spectra  due  to  compounds,  or  to  more 
complex  molecules,  which  might  form  near  the  condensing 
points  of  the  vapours. 


CELESTIAL  SPECTROSCOPY  409 

The  sun  and  stars  are  generally  regarded  as  consisting 
of  glowing  vapours  surrounded  by  a  photosphere  where 
condensation  is  taking  place,  the  temperature  of  the  photo- 
spheric  layer  from  which  the  greater  part  of  the  radiation 
comes  being  constantly  renewed  from  the  hotter  matter 
within. 

At  the  surface  the  convection  currents  would  be  too 
strong,  producing  a  considerable  commotion,  by  which  the 
different  gases  would  be  mixed  and  not  allowed  to  retain 
the  inequality  of  proportions  at  different  levels  due  to  their 
vapour  densities. 

Now,  the  conditions  of  the  radiating  photosphere  and 
those  of  the  gases  above  it,  on  which  the  character  of  the 
spectrum  of  a  star  depends,  will  be  determined,  not  alone 
by  the  temperature,  but  also  by  the  force  of  gravity  in  these 
regions;  this  force  will  be  fixed  by  the  star's  mass  and  its 
stage  of  condensation,  and  will  become  greater  as  the  star 
continues  to  condense. 

In  the  case  of  the  sun  the  force  of  gravity  has  already 
become  so  great  at  the  surface  that  the  decrease  of  the  den- 
sity of  the  bases  must  be  extremely  rapid,  passing  in  the 
space  of  a  few  miles  from  atmospheric  pressure  to  a  den- 
sity infinitesimally  small;  consequently  the  temperature 
gradient  at  the  surface,  if  determined  solely  by  expansion, 
must  be  extremely  rapid.  The  gases  here,  however,  are  ex- 
posed to  the  fierce  radiation  of  the  sun,  and  unless  wholly 
transparent  would  take  up  heat,  especially  if  any  solid  or 
liquid  particles  were  present  from  condensation  or  convec- 
tion currents. 

From  these  causes,  within  a  very  small  extent  of  space 
at  the  surface  of  the  sun,  all  bodies  with  which  we  are  ac- 
quainted should  fall  to  a  condition  in  which  the  extremely 
tenuous  gas  could  no  longer  give  a  visible  spectrum.  The 
insignificance  of  the  angle  subtended  by  this  space  as  seen 
from  the  earth  should  cause  the  boundary  of  the  solar  at- 
mosphere to  appear  defined.  If  the  boundary  which  we 
see  be  that  of  the  sun  proper,  the  matter  above  it  will  have 
to  be  regarded  as  in  an  essentially  dynamical  condition — 


4io  HUGGINS 

an  assemblage,  so  to  speak,  of  gaseous  projectiles  for  the 
most  part  falling  back  upon  the  sun  after  a  greater  or  less 
range  of  flight.  But  in  any  case  it  is  within  a  space  of  rela- 
tively small  extent  in  the  sun,  and  probably  in  the  other 
solar  stars,  that  the  reversion  which  is  manifested  by  dark 
lines  is  to  be  regarded  as  taking  place. 

Passing  backward  in  the  star's  life,  we  should  find  a 
gradual  weakening  of  gravity  at  the  surface,  a  reduction  of 
the  temperature  gradient  so  far  as  it  was  determined  by 
expansion,  and  convection  currents  of  less  violence  pro- 
ducing less  interference  with  the  proportional  quantities  of 
gases  due  to  their  vapour  densities,  while  the  effects  of 
eruptions  would  be  more  extensive. 

At  last  we  might  come  to  a  state  of  things  in  which,  if 
the  stars  were  hot  enough,  only  hydrogen  might  be  suf- 
ficiently cool  relatively  to  the  radiation  behind  to  produce 
a  strong  absorption.  The  lower  vapours  would  be  pro- 
tected, and  might  continue  to  be  relatively  too  hot  for  their 
lines  to  appear  very  dark  upon  the  continuous  spectrum; 
besides,  their  lines  might  be  possibly  to  some  extent  effaced 
by  the  coming  in  under  such  conditions  in  the  vapours 
themselves  of  a  continuous  spectrum. 

In  such  a  star  the  light  radiated  toward  the  upper  part 
of  the  atmosphere  may  have  come  from  portions  lower 
down  of  the  atmosphere  itself,  or  at  least  from  parts  not 
greatly  hotter.  There  may  be  no  such  great  difference  of 
temperature  of  the  low  and  less  low  portions  of  the  star's 
atmosphere  as  to  make  the  darkening  effect  of  absorption 
of  the  protected  metallic  vapours  to  prevail  over  the  illu- 
minating effect  of  their  emission. 

It  is  only  by  a  vibratory  motion  corresponding  to  a  very 
high  temperature  that  the  bright  lines  of  the  first  spectrum 
of  hydrogen  can  be  brought  out,  and  by  the  equivalence 
of  absorbing  and  emitting  power  that  the  corresponding 
spectrum  of  absorption  should  be  produced;  yet  for  a 
strong  absorption  to  show  itself,  the  hydrogen  must  be 
cool  relatively  to  the  source  of  radiation  behind  it,  whether 
this  be  condensed  particles  or  gas.  Such  conditions,  it 


CELESTIAL  SPECTROSCOPY  411 

seems  to  me,  should  occur  in  the  earlier  rather  than  in  the 
more  advanced  stages  of  condensation. 

The  subject  is  obscure,  and  we  may  go  wrong  in  our 
mode  of  conceiving  of  the  probable  progress  of  events,  but 
there  can  be  no  doubt  that  in  one  remarkable  instance  the 
white-star  spectrum  is  associated  with  an  early  stage  of 
condensation. 

Sirius  is  one  of  the  most  conspicuous  examples  of  one 
type  of  this  class  of  stars.  Photometric  observations,  com- 
bined with  its  ascertained  parallax,  show  that  this  star  emits 
from  forty  to  sixty  times  the  light  of  our  sun,  even  to  the 
eye,  which  is  insensible  to  ultra-violet  light,  in  which  Sirius 
is  very  rich,  while  we  learn  from  the  motion  of  its  compan- 
ion that  its  mass  is  not  much  more  than  double  that  of  our 
sun.  It  follows  that  unless  we  attribute  to  this  star  an 
improbably  great  emissive  power,  it  must  be  of  immense 
size,  and  in  a  much  more  diffuse  and  therefore  an  earlier 
condition  than  our  sun;  though  probably  at  a  later  stage 
than  those  white  stars  in  which  the  hydrogen  lines  are 
bright. 

A  direct  determination  of  the  relative  temperature  of  the 
photospheres  of  the  stars  might  possibly  be  obtained  in 
some  cases  from  the  relative  position  of  maximum  radia- 
tion of  their  continuous  spectra.  Langley  has  shown  that 
through  the  whole  range  of  temperature  on  which  we  can 
-experiment,  and  presumably  at  temperatures  beyond  the 
maximum  of  radiation,  power  in  solid  bodies  gradually 
shifts  upward  in  the  spectrum  from  the  infra-red  through 
the  red  and  orange,  and  that  in  the  sun  it  has  reached 
the  blue. 

The  defined  character,  as  a  rule,  of  the  stellar  lines  of 
absorption  suggests  that  the  vapours  producing  them  do 
not  at  the  same  time  exert  any  strong  power  of  general 
absorption.  Consequently  we  should  probably  not  go  far 
wrong,  when  the  photosphere  consists  of  liquid  or  solid 
particles,  if  we  could  compare  select  parts  of  the  continuous 
spectrum  between  the  stronger  lines  or  where  they  are  few- 
est. It  is  obvious  that  if  extended  portions  of  different 


4I2  HUGGINS 

stellar  spectra  were  compared,  their  true  relation  would  be 
obscured  by  the  line-absorption. 

The  increase  of  temperature,  as  shown  by  the  rise  in 
the  spectrum  of  the  maximum  of  radiation,  may  not  always 
be  accompanied  by  a  corresponding  greater  brightness  of 
a  star  as  estimated  by  the  eye,  which  is  an  extremely  im- 
perfect photometric  instrument.  Not  only  is  the  eye  blind 
to  large  regions  of  radiations,  but  even  for  the  small  range 
of  light  that  we  can  see,  the  visual  effect  varies  enormously 
with  its  colour.  According  to  Professor  Langley,  the  same 
amount  of  energy  which  just  enables  us  to  perceive  light 
in  the  crimson  at  A  would  in  the  green  produce  a  visual 
effect  100,000  times  greater.  In  the  violet  the  proportional 
effect  would  be  1,600,  in  the  blue  62,000,  in  the  yellow 
28,000,  in  the  orange  14,000,  and  in  the  red  1,200.  Cap- 
tain Abney's  recent  experiments  make  the  sensitiveness  of 
the  eye  for  the  green  near  F  to  be  750  times  greater  than 
for  red  about  C.  It  is  for  this  reason,  at  least  in  part,  that 
I  suggested  in  1864,  and  have  since  shown  by  direct  ob- 
servation, that  the  spectrum  of  the  nebula  in  Andromeda, 
and  presumably  of  similar  nebulae,  is  in  appearance  only 
wanting  in  the  red. 

The  stage  at  which  the  maximum  radiation  is  in  the 
green,  corresponding  to  the  eye's  greatest  sensitiveness, 
would  be  that  in  which  it  could  be  most  favourably  meas- 
ured by  eye-photometry.  As  the  maximum  rose  into  the 
violet  and  beyond,  the  star  would  increase  in  visual  bright- 
ness, but  not  in  proportion  to  the  increase  of  energy  radi- 
ated by  it. 

The  brightness  of  a  star  would  be  affected  by  the  nature 
of  the  substance  by  which  the  light  was  chiefly  emitted. 
In  the  laboratory  solid  carbon  exhibits  the  highest  emis- 
sive power.  A  stellar  stage  in  which  radiation  comes,  to  a 
large  extent,  from  a  photosphere  of  the  solid  particles  of 
this  substance  would  be  favourable  for  great  brilliancy. 
Though  the  stars  are  built  up  of  matter  essentially  similar 
to  that  of  the  sun,  it  does  not  follow  that  the  proportion 
of  the  different  elements  is  everywhere  the  same.  It  may 


CELESTIAL   SPECTROSCOPY  413 

be  that  the  substances  condensed  in  the  photospheres  of 
different  stars  may  differ  in  their  emissive  powers,  but  prob- 
ably not  to  a  great  extent. 

All  the  heavenly  bodies  are  seen  by  us  through  the 
tinted  medium  of  our  atmosphere.  According  to  Langley, 
the  solar  stage  of  stars  is  not  really  yellow,  but,  even  as 
gauged  by  our  imperfect  eyes,  would  appear  bluish-white 
if  we  could  free  ourselves  from  the  deceptive  influences  of 
our  surroundings. 

From  these  considerations  it  follows  that  we  can  scarcely 
infer  the  evolutional  stages  of  the  stars  from  a  simple  com- 
parison of  their  eye-magnitudes.  We  should  expect  the 
white  stars  to  be,  as  a  class,  less  dense  than  the  stars  in  the 
solar  stage.  As  great  mass  might  bring  in  the  solar  type 
of  spectrum  at  a  relatively  earlier  time,  some  of  the  bright- 
est of  these  stars  may  be  very  massive  and  brighter  than 
the  sun — for  example,  the  brilliant  star  Arcturus.  For  these 
reasons  the  solar  stars  should  not  only  be  denser  than  the 
white  stars,  but  perhaps,  as  a  class,  surpass  them  in  mass 
and  eye-brightness. 

It  has  been  shown  by  Lane  that,  so  long  as  a  conden- 
sing gaseous  mass  remains  subject  to  the  laws  of  a  purely 
gaseous  body,  its  temperature  will  continue  to  rise. 

The  greater  or  less  breadth  of  the  lines  of  absorption 
of  hydrogen  in  the  white  stars  may  be  due  to  variations 
of  the  depth  of  the  hydrogen  in  the  line  of  sight,  arising 
from  the  causes  which  have  been  discussed.  At  the  sides 
of  the  lines  the  absorption  and  emission  are  feebler  than 
in  the  middle,  and  would  come  out  more  strongly  with  a 
greater  thickness  of  gas. 

The  diversities  among  the  white  stars  are  nearly  as  nu- 
merous as  the  individuals  of  the  class.  Time  does  not 
permit  me  to  do  more  than  to  record  that  in  addition  to 
the  three  subclasses  into  which  they  have  been  divided  by 
Vogel,  Scheiner  has  recently  investigated  minor  differences 
as  suggested  by  the  character  of  the  third  line  of  hydrogen 
near  G.  He  has  pointed  out,  too,  that  so  far  as  his  observa- 
tions go  the  white  stars  in  the  constellation  of  Orion  stand 


414  HUGGINS 

alone,  with  the  exception  of  Algol,  in  possessing  a  dark  line 
in  the  blue  which  has  apparently  the  same  position  as  a 
bright  line  in  the  great  nebula  of  the  same  constellation; 
and  Pickering  finds  in  his  photographs  of  the  spectra  of 
the  stars  dark  lines  corresponding  to  the  principal  lines 
of  the  bright-line  stars,  and  the  planetary  nebulae  with  the 
exception  of  the  chief  nebular  line.  The  association  of 
white  stars  with  nebular  matter  in  Orion,  in  the  Pleiades, 
in  the  region  of  the  Milky  Way,  and  in  other  parts  of  the 
heavens,  may  be  regarded  as  falling  in  with  the  view  that 
I  have  taken. 

In  the  stars  possibly  farther  removed  from  the  white 
class  than  our  sun,  belonging  to  the  first  division  of  Vogel's 
third  class,  which  are  distinguished  by  absorption  bands 
with  their  stronger  edge  toward  the  blue,  the  hydrogen  lines 
are  narrower  than  in  the  solar  spectrum.  In  these  stars  the 
density-gradient  is  probably  still  more  rapid,  the  depth  of 
hydrogen  may  be  less,  and  possibly  the  hydrogen  molecules 
may  be  affected  by  a  larger  number  of  encounters  with  dis- 
similar molecules.  In  some  red  stars  with  dark  hydro- 
carbon bands  the  hydrogen  lines  have  not  been  certainly 
observed;  if  they  are  really  absent  it  may  be  because  the 
temperature  has  fallen  below  the  point  at  which  hydrogen 
can  exert  its  characteristic  absorption;  besides,  some  hydro- 
gen will  have  united  with  the  carbon.  The  coming  in  of 
the  hydrocarbon  bands  may  indicate  a  later  evolutional 
stage,  but  the  temperature  may  still  be  high,  as  acetylene 
can  exist  in  the  electric  arc. 

A  number  of  small  stars  more  or  less  similar  to  those 
which  are  known  by  the  names  of  their  discoverers,  Wolf 
and  Rayet,  have  been  found  by  Pickering  in  his  photo- 
graphs. These  are  remarkable  for  several  brilliant  groups 
of  bright  lines,  including  frequently  the  hydrogen  lines  and 
the  line  D3,  upon  a  continuous  spectrum  strong  in  blue 
and  violet  rays,  in  which  are  also  dark  lines  of  absorption. 
As  some  of  the  bright  groups  appear  in  his  photographs 
to  agree  in  position  with  corresponding  bright  lines  in 
the  planetary  nebulae,  Pickering  suggests  that  these  stars 


CELESTIAL  SPECTROSCOPY  415 

should  be  placed  in  one  class  with  them,  although  the 
brightest  nebular  line  is  absent  from  these  stars.  The  sim- 
plest conception  of  their  nature  would  be  that  each  star  is 
surrounded  by  a  nebula,  the  bright  groups  being  due  to 
gaseous  matter  outside  the  star.  Mr.  Roberts,  however, 
has  not  been  able  to  bring  out  any  indication  of  nebulosity 
by  prolonged  exposure.  The  remarkable  star  rj  Argus  may 
belong  to  this  class  of  the  heavenly  bodies. 

In  the  nebulae  the  elder  Herschel  saw  portions  of  the 
fiery  mist  or  "  shining  fluid  "  out  of  which  the  heavens  and 
the  earth  had  been  slowly  fashioned.  For  a  time  this  view 
of  the  nebulae  gave  place  to  that  which  regarded  them  as 
external  galaxies,  cosmical  "  sand  heaps,"  too  remote  to  be 
resolved  into  separate  stars;  though,  indeed,  in  1858,  Mr. 
Herbert  Spencer  showed  that  the  observations  of  nebulae 
up  to  that  time  were  really  in  favour  of  an  evolutional 
progress. 

In  1864  I  brought  the  spectroscope  to  bear  upon  them; 
the  bright  lines  which  flashed  upon  the  eye  showed  the 
source  of  the  light  of  a  number  of  them  to  be  glowing  gas, 
and  so  restored  these  bodies  to  what  is  probably  their  true 
place,  as  an  early  stage  of  sidereal  life. 

At  that  early  time  our  knowledge  of  stellar  spectra  was 
small.  For  this  reason,  partly,  and  probably  also  under  the 
undue  influence  of  theological  opinions  then  widely  preva- 
lent, I  unwisely  wrote  in  my  original  papers  in  1864  that 
"  in  these  subjects  we  no  longer  have  to  do  with  a  special 
modification  of  our  own  type  of  sun,  but  find  ourselves  in 
presence  of  objects  possessing  a  distinct  and  peculiar  plan 
of  structure."  Two  years  later,  however,  in  a  lecture  before 
this  association,  I  took  a  truer  position.  "  Our  views  of 
the  universe,"  I  said,  "  are  undergoing  important  changes; 
let  us  wait  for  more  facts,  with  minds  unfettered  by  any 
dogmatic  theory,  and  therefore  free  to  receive  the  obvious 
teaching,  whatever  it  may  be,  of  new  observations." 

Let  us  turn  aside  for  a  moment  from  the  nebulae  in  the 
sky  to  the  conclusions  to  which  philosophers  had  been 
irresistibly  led  by  a  consideration  of  the  features  of  the  solar 


4I6  HUGGINS 

system.  We  have  before  us  in  the  sun  and  planets  obvi- 
ously not  a  haphazard  aggregation  of  bodies,  but  a  system 
resting  upon  a  multitude  of  relations  pointing  to  a  common 
physical  cause.  From  these  considerations  Kant  and  La- 
place formulated  the  nebular  hypothesis,  resting  it  on  gravi- 
tation alone,  for  at  that  time  the  science  of  the  conservation 
of  energy  was  practically  unknown.  These  philosophers 
showed  how,  on  the  supposition  that  the  space  now  occu- 
pied by  the  solar  system  was  once  filled  by  a  vaporous  mass, 
the  formation  of  the  sun  and  planets  could  be  reasonably 
accounted  for. 

By  a  totally  different  method  of  reasoning,  modern  sci- 
ence traces  the  solar  system  backward  step  by  step  to  a 
similar  state  of  things  at  the  beginning.  According  to 
Helmholtz,  the  sun's  heat  is  maintained  by  the  contraction 
of  his  mass,  at  the  rate  of  about  220  feet  a  year.  Whether 
at  the  present  time  the  sun  is  getting  hotter  or  colder  we 
do  not  certainly  know.  We  can  reason  back  to  the  time 
when  the  sun  was  sufficiently  expanded  to  fill  the  whole 
space  occupied  by  the  solar  system,  and  was  reduced  to  a 
great  glowing  nebula.  Though  man's  life,  the  life  of  the 
race,  perhaps,  is  too  short  to  give  us  direct  evidence  of  any 
distinct  stages  of  so  august  a  process,  still  the  probability  is 
great  that  the  nebular  hypothesis,  especially  in  the  more 
precise  form  given  to  it  by  Roche,  does  express  broadly, 
notwithstanding  some  difficulties,  the  succession  of  events 
through  which  the  sun  and  planets  have  passed. 

The  nebular  hypothesis  of  Laplace  requires  a  rotating 
mass  of  fluid  which  at  successive  epochs  became  unstable 
from  excess  of  motion,  and  left  behind  rings,  or  more  prob- 
ably perhaps  lumps  of  matter,  from  the  equatorial  regions. 

The  difficulties  to  which  I  have  referred  have  suggested 
to  some  thinkers  a  different  view  of  things,  according  to 
which  it  is  not  necessary  to  suppose  that  one  part  of  the 
system  gravitationally  supports  another.  The  whole  may 
consist  of  a  congeries  of  discrete  bodies,  even  if  these  bodies 
be  the  ultimate  molecules  of  matter.  The  planets  may  have 
been  formed  by  the  gradual  accretion  of  such  discrete 


CELESTIAL  SPECTROSCOPY  417 

bodies.  On  the  view  that  the  material  of  the  condensing 
solar  system  consisted  of  separate  particles  or  masses,  we 
have  no  longer  the  fluid  pressure  which  is  an  essential  part 
of  Laplace's  theory.  Faye,  in  his  theory  of  evolution  from 
meteorites,  has  to  throw  over  this  fundamental  idea  of  the 
nebular  hypothesis,  and  he  formulates  instead  a  different 
succession  of  events  in  which  the  outer  planets  were  formed 
last — a  theory  which  has  difficulties  of  its  own. 

Professor  George  Darwin  has  recently  shown,  from  an 
investigation  of  the  mechanical  condition  of  a  swarm  of 
meteorites,  that  on  certain  assumptions  a  meteoric  swarm 
might  behave  as  a  coarse  gas,  and  in  this  way  bring  back 
the  fluid  pressure  exercised  by  one  part  of  the  system  on 
the  other,  which  is  required  by  Laplace's  theory.  Our  chief 
assumption  consists  in  supposing  that  such  inelastic  bodies 
as  meteoric  stones  might  attain  the  effective  elasticity  of  a 
high  order  which  is  necessary  to  the  theory  through  the 
sudden  volatilization  of  a  part  of  their  mass  at  an  encoun- 
ter, by  which  what  is  virtually  a  violent  explosive  is  intro- 
duced between  the  two  colliding  stones.  Professor  Darwin 
is  careful  to  point  out  that  it  must  necessarily  be  obscure  as 
to  how  small  a  mass  of  solid  matter  can  take  up  a  very  large 
amount  of  energy  in  a  small  fraction  of  a  second. 

Any  direct  indications  from  the  heavens  themselves, 
however  slight,  are  of  so  great  value  that  I  should  perhaps 
in  this  connection  call  attention  to  a  recent  remarkable 
photograph  by  Mr.  Roberts  of  the  great  nebula  in  Androm- 
eda. On  this  plate  we  seem  to  have  presented  to  us  some 
stage  of  cosmical  evolution  on  a  gigantic  scale.  The  photo- 
graph shows  a  sort  of  whirlpool  disturbance  of  the  luminous 
matter  which  is  distributed  in  a  plane  inclined  to  the  line 
of  sight,  in  which  a  series  of  rings  of  bright  matter  separated 
by  dark  spaces,  greatly  foreshortened  by  perspective,  sur- 
round a  large  undefined  central  mass.  The  parallax  of  this 
nebula  has  not  been  ascertained,  but  there  can  be  little 
doubt  that  we  are  looking  upon  a  system  very  remote,  and 
therefore  of  a  magnitude  great  beyond  our  power  of  ade- 
quate comprehension.  The  matter  of  this  nebula,  in  what- 
27 


418  HUGGINS 

ever  state  it  may  be,  appears  to  be  distributed,  as  in  so 
many  other  nebulae,  in  rings  or  spiral  streams,  and  to  sug- 
gest a  stage  in  a  succession  of  evolutional  events  not  in- 
consistent with  that  which  the  nebular  hypothesis  requires. 
To  liken  this  object  more  directly  to  any  particular  stage 
in  the  formation  of  the  solar  system  would  be  to  compare 
things  great  with  small,  and  might  be  indeed  to  introduce 
a  false  analogy;  but,  on  the  other  hand,  we  should  err 
through  an  excess  of  caution  if  we  did  not  accept  the  re- 
markable features  brought  to  light  by  this  photograph  as 
a  presumptive  indication  of  a  progress  of  events  in  cos- 
mical  history  following  broadly  upon  the  lines  of  Laplace's 
theory. 

The  old  view  of  the  original  matter  of  the  nebulae,  that 
it  consisted  of  a  "  fiery  mist  " — 

"...  a  tumultuous  cloud 
Instinct  with  fire  and  'nitre  " — 

fell  at  once  with  the  rise  of  the  science  of  thermodynamics. 
In  1854  Helmholtz  showed  that  the  supposition  of  an 
original  fiery  condition  of  the  nebulous  stuff  was  unneces- 
sary, since  in  the  mutual  gravitation  of  widely  separated 
matter  we  have  a  store  of  potential  energy  sufficient  to 
generate  the  high  temperature  of  the  sun  and  stars.  We 
can  scarcely  go  wrong  in  attributing  the  light  of  the  nebulae 
to  the  conversion  of  the  gravitational  energy  of  shrinkage 
into  molecular  motion. 

The  idea  that  the  light  of  comets  and  of  nebulae  may 
be  due  to  a  succession  of  ignited  flashes  of  gas  from  the 
encounters  of  meteoric  stones  was  suggested  by  Professor 
Tait,  and  was  brought  to  the  notice  of  this  association  in 
1871  by  Sir  William  Thomson  in  his  presidential  address. 

The  spectrum  of  the  bright-line  nebulae  is  certainly  not 
such  a  spectrum  as  we  should  expect  from  the  flashing  by 
collisions  of  meteorites  similar  to  those  which  have  been 
analyzed  in  our  laboratories.  The  strongest  lines  of  the 
substances  which  in  the  case  of  such  meteorites  would  first 
show  themselves — iron,  sodium,  magnesium,  nickel,  etc. — 


CELESTIAL   SPECTROSCOPY  419 

are  not  those  which  distinguish  the  nebular  spectrum.  On 
the  contrary,  this  spectrum  is  chiefly  remarkable  for  a  few 
brilliant  lines,  very  narrow  and  denned  upon  a  background 
of  a  faint  continuous  spectrum,  which  contains  numerous 
bright  lines,  and  probably  some  lines  of  absorption. 

The  two  most  conspicuous  lines  have  not  been  inter- 
preted; for  though  the  second  line  falls  near,  it  is  not  coin- 
cident with  a  strong  double  line  of  iron.  It  is  hardly  neces- 
sary to  say  that  though  the  near  position  of  the  brightest 
line  to  the  bright  double  line  of  nitrogen,  as  seen  in  a  small 
spectroscope  in  1864,  naturally  suggested  at  that  early  time 
the  possibility  of  the  presence  of  this  element  in  the  nebulae, 
I  have  been  careful  to  point  out,  to  prevent  misapprehen- 
sion, that  in  more  recent  years  the  nitrogen  line  and  subse- 
quently a  lead  line  have  been  employed  by  me  solely  as 
fiducial  points  of  reference  in  the  spectrum. 

The  third  line  we  know  to  be  the  second  line  of  the 
first  spectrum  of  hydrogen.  Mr.  Keeler  has  seen  the  first 
hydrogen  line  in  the  red,  and  photographs  show  that  this 
hydrogen  spectrum  is  probably  present  in  its  complete  form, 
or  nearly  so,  as  we  first  learned  to  know  it  in  the  absorption 
spectrum  of  the  white  stars. 

We  are  not  surprised  to  find  associated  with  it  the  line 
D3  near  the  position  of  the  absent  sodium  lines,  probably 
due  to  the  atom  of  some  unknown  gas,  which  in  the  sun 
can  only  show  itself  in  the  outbursts  of  highest  tempera- 
ture, and  for  this  reason  does  not  reveal  itself  by  absorption 
in  the  solar  spectrum. 

It  is  not  unreasonable  to  assume  that  the  two  bright- 
est lines,  which  are  of  the  same  order  as  the  third  line,  are 
produced  by  substances  of  a  similar  nature,  in  which  a 
vibratory  motion  corresponding  to  a  very  high  temperature 
is  also  necessary.  These  substances,  as  well  as  that  repre- 
sented by  the  line  D3,  may  be  possibly  some  of  the  un- 
known elements  which  are  wanting  in  our  terrestrial  chem- 
istry between  hydrogen  and  lithium,  unless,  indeed,  D3  be 
on  the  lighter  side  of  hydrogen. 

In  the  laboratory  we  must  have  recourse  to  the  elec- 


420  HUGGINS 

trie  discharge  to  bring  out  the  spectrum  of  hydrogen;  but 
in  a  vacuum  tube,  though  the  radiation  may  be  great,  from 
the  relative  fewness  of  the  luminous  atoms  or  molecules  or 
from  some  other  cause,  the  temperature  of  the  gas  as  a 
whole  may  be  low. 

On  account  of  the  large  extent  of  the  nebulae,  a  com- 
paratively small  number  of  luminous  molecules  or  atoms 
would  probably  be  sufficient  to  make  the  nebulas  as  bright 
as  they  appear  to  us.  On  such  an  assumption  the  average 
temperature  may  be  low,  but  the  individual  particles,  which 
by  their  encounters  are  luminous,  must  have  motions  cor- 
responding to  a  very  high  temperature,  and  in  this  sense 
be  extremely  hot. 

In  such  diffuse  masses,  from  the  great  mean  length  of 
free  path,  the  encounters  would  be  rare  but  correspond- 
ingly violent,  and  tend  to  bring  about  vibrations  of  com- 
paratively short  period,  as  appears  to  be  the  case  if  we  may 
judge  by  the  great  relative  brightness  of  the  more  re- 
frangible lines  of  the  nebular  spectrum. 

Such  a  view  may  perhaps  reconcile  the  high  tempera- 
ture which  the  nebular  spectrum  undoubtedly  suggests  with 
the  much  lower  mean  temperature  of  the  gaseous  mass, 
which  we  should  expect  at  so  early  a  stage  of  condensation, 
unless  we  assume  a  very  enormous  mass ;  or  that  the  matter 
coming  together  had  previously  considerable  motion,  or 
considerable  molecular  agitation. 

If  the  hydrogen  shown  by  the  spectroscope  in  the 
nebulae  and  in  the  atmospheres  of  the  stars  is  retained  by 
the  bodies,  we  should  be  able  to  assign  approximately  an 
inferior  limit  for  the  force  of  gravity  at  their  surfaces,  pro- 
vided we  assume  that  the  gas  is  in  the  uncombined  state, 
and  always  exists  in  some  greater  proportion  than  in  the 
free  space  about  them. 

The  inquisitiveness  of  the  human  mind  does  not  allow 
us  to  remain  content  with  the  interpretation  of  the  present 
state  of  the  cosmical  masses,  but  suggests  the  question — 

"  What  see'st  thou  else 
In  the  dark  backward  and  abysm  of  time?  " 


CELESTIAL  SPECTROSCOPY 


421 


What  was  the  original  state  of  things?  how  has  it  come 
about  that  by  the  side  of  aging  worlds  we  have  nebula 
in  a  relatively  younger  stage?  Have  any  of  them  received 
their  birth  from  dark  suns,  which  have  collided  into  new 
life,  and  so  belong  to  a  second  or  later  generation  of  the 
heavenly  bodies? 

During  the  short  historic  period,  however,  there  is  no 
record  of  such  an  event;  still,  it  would  seem  to  be  only 
through  the  collision  of  dark  suns,  of  which  the  number 
must  be  increasing,  that  a  temporary  rejuvenescence  of 
the  heavens  is  possible,  and  by  such  ebbings  and  flowings 
of  stellar  life  that  the  inevitable  end  to  which  evolution  in 
its  apparently  uncompensated  progress  is  carrying  us  can, 
even  for  a  little,  be  delayed. 

We  can  not  refuse  to  admit  as  possible  such  an  origin 
for  nebulae.  In  considering,  however,  the  formation  of  the 
existing  nebulae,  we  must  bear  in  mind  that,  in  the  part  of 
the  heavens  within  our  ken,  the  stars  still  in  the  early  and 
middle  stages  of  evolution  exceed  greatly  in  number  those 
which  appear  to  be  in  an  advanced  condition  of  condensa- 
tion. Indeed,  we  find  some  stars  which  may  be  regarded 
as  not  far  advanced  beyond  the  nebular  condition. 

It  may  be  that  the  cosmical  bodies  which  are  still 
nebulous  owe  the  lateness  of  their  development  to  some 
conditions  of  the  part  of  space  where  they  occur,  such  as 
conceivably  a  greater  original  homogeneity,  in  consequence 
of  which  condensation  began  less  early.  In  other  parts  of 
space  condensation  may  have  been  still  further  delayed  or 
even  have  not  yet  begun.  It  is  worthy  of  remark  that  these 
nebulae  group  themselves  about  the  Milky  Way,  where  we 
find  a  preponderance  of  the  white-star  type  of  stars,  and 
almost  exclusively  the  bright-line  stars  which  Pickering 
associates  with  the  planetary  nebulae.  Further,  Dr.  Gill 
concludes,  from  the  rapidity  with  which  they  impress  them- 
selves upon  the  plate,  that  the  fainter  stars  of  the  Milky 
\Vay  also,  to  a  large  extent,  belong  to  this  early  type  of 
stars.  At  the  same  time  other  types  of  stars  occur  also 
over  this  region,  and  the  red  hydrocarbon  stars  are  found 


422  MUGGINS 

in  certain  parts:  but  possibly  these  stars  may  be  before 
or  behind  the  Milky  Way,  and  not  physically  connected 
with  it. 

If  light  matter  be  suggested  by  the  spectrum  of  these 
nebulae,  it  may  be  asked  further,  as  a  pure  speculation, 
whether  in  them  we  are  witnessing  possibly  a  later  con- 
densation of  the  light  matter  which  had  been  left  behind, 
at  least  in  a  relatively  greater  proportion,  after  the  first 
growth  of  the  worlds  into  which  the  heavier  matter  con- 
densed, though  not  without  some  entanglement  of  the 
higher  substances.  The  wide  extent  and  great  diffuseness 
of  this  bright-line  nebulosity  over  a  large  part  of  the  con- 
stellation of  Orion  may  be  regarded  perhaps  as  pointing 
in  this  direction.  The  diffuse  nebulous  matter  streaming 
round  the  Pleiades  may  possibly  be  another  instance, 
though  the  character  of  its  spectrum  has  not  yet  been 
ascertained. 

In  the  planetary  nebulae,  as  a  rule,  there  is  a  sensible  in- 
crease of  the  faint  continuous  spectrum,  as  well  as  a  slight 
thickening  of  the  bright  lines  toward  the  centre  of  the 
nebula,  appearances  which  are  in  favour  of  the  view  that 
these  bodies  are  condensing  gaseous  masses. 

Professor  George  H.  Darwin,  in  his  investigation  of  the 
equilibrium  of  a  rotating  mass  of  fluid,  found,  in  accord- 
ance with  the  independent  researches  of  Poincare,  that  when 
a  portion  of  the  centra"!  body  becomes  detached  through 
increasing  angular  velocity,  the  portion  should  bear  a  far 
larger  ratio  to  the  remainder  than  is  observed  in  the  planets 
and  satellites  of  the  solar  system;  even  taking  into  account 
heterogeneity  from  the  condensation  of  the  parent  mass. 

Now  this  state  of  things,  in  which  the  masses,  though 
not  equal,  are  of  the  same  order,  does  seem  to  prevail  in 
many  nebulae,  and  to  have  given  birth  to  a  large  class  of 
binary  stars.  Mr.  See  has  recently  investigated  the  evolu- 
tion of  bodies  of  this  class,  and  points  out  their  radical  dif- 
ferences from  the  solar  system  in  the  relatively  large  mass- 
ratios  of  the  component  bodies,  as  well  as  the  high  eccen- 
tricities of  their  orbits  brought  about  by  tidal  friction  which 


CELESTIAL   SPECTROSCOPY  423 

would  play  a  more  important  part  in  the  evolution  of  such 
a  system.  Considering  the  large  number  of  these  bodies,  he 
suggests  that  the  solar  system  should  perhaps  no  longer 
be  regarded  as  representing  celestial  evolution  in  its  normal 
form — 

"  A  goodly  Paterne  to  whose  perfect  mould 
He  fashioned  them    .  .  ." 

but  rather  as  modified  by  conditions  which  are  exceptional. 

It  may  well  be  that  in  the  very  early  stages  condensing 
masses  are  subject  to  very  different  conditions,  and  that 
condensation  may  not  always  begin  at  one  or  two  centres, 
but  sometimes  set  in  at  a  large  number  of  points,  and  pro- 
ceed in  the  different  cases  along  very  different  lines  of  evo- 
lution. Besides  its  more  direct  use  in  the  chemical  analysis 
of  the  heavenly  bodies,  the  spectroscope  has  given  to  us  a 
great  and  unexpected  power  of  advance  along  the  lines  of 
the  older  astronomy.  In  the  future  a  higher  value  may, 
indeed,  be  placed  upon  this  indirect  use  of  the  spectroscope 
than  upon  its  chemical  revelations. 

By  no  direct  astronomical  methods  could  motions  of 
approach  or  of  recession  of  the  stars  be  even  detected,  much 
less  could  they  be  measured.  A  body  coming  directly  to- 
ward us  or  going  directly  from  us  appears  to  stand  still.  In 
the  case  of  the  stars  we  can  receive  no  assistance  from 
change  of  size  or  of  brightness.  The  stars  show  no  true 
disks  in  our  instruments,  and  the  nearest  of  them  is  so 
far  off  that  if  it  were  approaching  us  at  the  rate  of  a  hundred 
miles  in  a  second  of  time,  a  whole  century  of  such  rapid 
approach  would  not  do  more  than  increase  its  brightness  by 
the  one-fortieth  part. 

Still,  it  was  only  too  clear  that,  so  long  as  we  were  un- 
able to  ascertain  directly  those  components  of  the  stars' 
motions  which  lie  in  the  line  of  sight,  the  speed  and  direc- 
tion of  the  solar  motion  in  space,  and  many  of  the  great 
problems  of  the  constitution  of  the  heavens,  must  remain 
more  or  less  imperfectly  known.  Now,  the  spectroscope 
has  placed  in  our  hands  this  power,  which,  though  so  essen- 
tial, appeared  almost  in  the  nature  of  things  to  lie  forever 


424  MUGGINS 

beyond  our  grasp;  it  enables  us  to  measure  directly  and 
under  favourable  circumstances  to  within  a  mile  per  sec- 
ond, or  even  less,  the  speed  of  approach  or  of  recession  of 
a  heavenly  body.  This  method  of  observation  has  the  great 
advantage  for  the  astronomer  of  being  independent  of  the 
distance  of  the  moving  body,  and  is  therefore  as  applicable 
and  as  certain  in  the  case  of  a  body  on  the  extreme  confines 
of  the  visible  universe,  so  long  as  it  is  bright  enough,  as  in 
the  case  of  a  neighbouring  planet. 

Doppler  had  suggested  as  far  back  as  1841  that  the 
same  principle,  on  which  he  had  shown  that  a  sound  should 
become  sharper  or  flatter  if  there  were  an  approach  or  a 
recession  between  the  ear  and  the  source  of  the  sound, 
would  apply  equally  to  light;  and  he  went  on  to  say  that 
the  difference  of  colour  of  some  of  the  binary  stars  might 
be  produced  in  this  way  by  their  motions.  Doppler  was 
right  in  that  the  principle  is  true  in  the  case  of  light,  but 
he  was  wrong  in  the  particular  conclusion  which  he  drew 
from  it.  Even  if  we  suppose  a  star  to  be  moving  with  a 
sufficiently  enormous  velocity  to  alter  sensibly  its  colour  to 
the  eye,  no  such  change  would  actually  be  seen,  for  the 
reason  that  the  store  of  invisible  light  beyond  both  limits 
of  the  visible  spectrum,  the  blue  and  the  red,  would  be 
drawn  upon,  and  the  light-waves  invisible  to  us  would  be 
exalted  or  degraded  so  as  to  take  the  place  of  those  raised 
or  lowered  in  the  visible  region,  and  the  colour  of  the  star 
would  remain  unchanged.  About  eight  years  later  Fizeau 
pointed  out  the  importance  of  considering  the  individual 
wave-lengths  of  which  white  light  is  composed.  It  is,  in- 
deed, Doppler's  principle  which  underlies  the  early  deter- 
mination of  the  velocity  of  light  by  Roemer;  but  this 
method,  in  its  converse  form,  can  scarcely  be  regarded  as 
of  practical  value  for  the  motions  in  the  line  of  sight  of 
binary  stars.  As  soon,  however,  as  we  had  learned  to  rec- 
ognise the  lines  of  known  substances  in  the  spectra  of  the 
heavenly  bodies,  Doppler's  principle  became  applicable  as 
the  basis  of  a  new  and  most  fruitful  method  of  investigation. 
The  measurement  of  the  small  shift  of  the  celestial  lines 


CELESTIAL  SPECTROSCOPY  425 

from  their  true  positions,  as  shown  by  the  same  lines  in  the 
spectrum  of  a  terrestrial  substance,  gives  to  us  the  means 
of  ascertaining  directly  in  miles  per  second  the  speed  of 
approach  or  of  recession  of  the  heavenly  body  from  which 
the  light  has  come. 

An  account  of  the  first  application  of  this  method  of  re- 
search to  the  stars,  which  was  made  in  my  observatory  in 
1868,  was  given  by  Sir  Gabriel  Stokes  from  this  chair  at  the 
meeting  at  Exeter  in  1869.  The  stellar  motions  deter- 
mined by  me  were  afterward  confirmed  by  Professor  Vogel 
in  the  case  of  Sirius,  and  in  the  case  of  other  stars  by 
Mr.  Christie,  now  Astronomer  Royal,  at  Greenwich;  but 
necessarily,  in  consequence  of  the  inadequacy  of  the  instru- 
ments then  in  use  for  so  delicate  an  inquiry,  the  amounts  of 
these  motions  were  but  approximate. 

The  method  was  shortly  afterward  taken  up  systematic- 
ally at  Greenwich  and  at  the  Rugby  Observatory.  It  is  to 
be  greatly  regretted  that,  for  some  reasons,  the  results  have 
not  been  sufficiently  accordant  and  accurate  for  a  research 
of  such  exceptional  delicacy.  On  this  account,  probably, 
as  well  as  that  the  spectroscope  at  that  early  time  had 
scarcely  become  a  familiar  instrument  in  the  observatory, 
astronomers  were  slow  in  availing  themselves  of  this  new 
and  remarkable  power  of  investigation.  That  this  com- 
parative neglect  of  so  truly  wonderful  a  method  of  ascer- 
taining what  was  otherwise  outside  our  powers  of  observa- 
tion has  greatly  retarded  the  progress  of  astronomy  during 
the  last  fifteen  years  is  but  too  clearly  shown  by  the  bril- 
liant results  which  within  the  last  two  years  have  followed 
fast  upon  the  recent  masterly  application  of  this  method  by 
photography  at  Potsdam,  and  by  eye  with  the  needful  ac- 
curacy at  the  Lick  Observatory.  At  last  this  use  of  the 
spectroscope  has  taken  its  true  place  as  one  of  the  most 
potent  methods  of  astronomical  research.  It  gives  us  the 
motions  of  approach  and  of  recession,  not  in  angular  meas- 
ures, which  depend  for  their  translation  into  actual  veloci- 
ties upon  separate  determinations  of  parallactic  displace- 
ments, but  at  once  in  terrestrial  units  of  distance. 


426  HUGGINS 

This  method  of  work  will  doubtless  be  very  prominent 
in  the  astronomy  of  the  near  future,  and  to  it  probably  we 
shall  have  to  look  for  the  more  important  discoveries  in 
sidereal  astronomy  which  will  be  made  during  the  coming 
century. 

In  his  recent  application  of  photography  to  this  method 
of  determining  celestial  motions,  Professor  Vogel,  assisted 
by  Dr.  Scheiner,  considering  the  importance  of  obtaining 
the  spectrum  of  as  many  stars  as  possible  on  an  extended 
scale  without  an  exposure  inconveniently  long,  wisely  de- 
termined to  limit  the  part  of  the  spectrum  on  the  plate  to 
the  region  for  which  the  ordinary  silver-bromide  gelatine 
plates  are  most  sensitive — namely,  to  a  small  distance  on 
each  side  of  G,  and  to  employ  as  the  line  of  comparison  the 
hydrogen  line  near  G,  and  recently  also  certain  lines  of  iron. 
The  most  minute  and  complete  mechanical  arrangements 
were  provided  for  the  purpose  of  securing  the  absolute 
rigidity  of  the  comparison  spectrum  relatively  to  that  of 
the  star,  and  for  permitting  temperature  adjustments  and 
other  necessary  ones  to  be  made. 

The  perfection  of  these  spectra  is  shown  by  the  large 
number  of  lines,  no  fewer  than  250  in  the  case  of  Capella, 
within  the  small  region  of  the  spectrum  on  the  plate. 
Already  the  motions  of  about  fifty  stars  have  been  meas- 
ured with  an  accuracy,  in  the  case  of  the  large  number  of 
them,  of  about  an  English  mile  per  second. 

At  the  Lick  Observatory  it  has  been  shown  that  ob- 
servations can  be  made  directly  by  eye  with  an  accuracy 
equally  great.  Mr.  Keeler's  brilliant  success  has  followed 
in  great  measure  from  the  use  of  the  third  and  fourth 
spectra  of  a  grating  with  14,438  lines  to  the  inch.  The 
marvellous  accuracy  attainable  in  his  hands  on  a  suitable 
star  is  shown  by  observations  on  three  nights  of  the  star 
Arcturus,  the  greatest  divergence  of  his  measures  being 
not  greater  than  six  tenths  of  a  mile  per  second,  while  the 
mean  of  the  three  nights'  work  agreed  with  the  mean  of 
five  photographic  determinations  of  the  same  star  at  Pots- 
dam to  within  one  tenth  of  an  English  mile.  These  are 


CELESTIAL  SPECTROSCOPY  427 

determinations  of  the  motions  of  a  sun  so  stupendously 
remote  that  even  the  method  of  parallax  practically  fails 
to  fathom  the  depth  of  intervening  space,  and  by  means  of 
light-waves  which  have  been,  according  to  Elkin's  nominal 
parallax,  nearly  two  hundred  years  upon  their  journey. 

Mr.  Keeler  with  his  magnificent  means  has  accom- 
plished a  task  which  I  attempted  in  vain  in  1874,  with  the 
comparatively  poor  appliances  at  my  disposal,  of  measuring 
the  motions  in  the  line  of  sight  of  some  of  the  planetary 
nebulae.  As  the  stars  have  considerable  motions  in  space, 
it  was  to  be  expected  that  nebulas  should  possess  similar 
motions,  for  the  stellar  motions  must  have  belonged  to  the 
nebulae  out  of  which  they  have  been  evolved.  My  instru- 
mental means,  limiting  my  power  of  detection  to  motions 
greater  than  twenty-five  miles  per  second,  were  insufficient. 
Mr.  Keeler  has  found  in  the  examination  of  ten  nebulas 
motions  varying  from  two  miles  to  twenty-seven  miles,  with 
one  exceptional  motion  of  nearly  forty  miles. 

For  the  nebula  of  Orion,  Mr.  Keeler  finds  a  motion  of 
recession  of  about  ten  miles  a  second.  Now,  this  motion 
agrees  with  what  it  should  appear  to  have  from  the  drift  of 
the  solar  system  itself,  so  far  as  it  has  been  possible  at  pres- 
ent to  ascertain  the  probable  velocity  of  the  sun  in  space. 
This  grand  nebula,  of  vast  extent  and  of  extreme  tenuity,  is 
probably  more  nearly  at  rest  relatively  to  the  stars  of  our 
system  than  any  other  celestial  object  we  know;  still,  it 
would  seem  more  likely  that  even  here  we  have  some  mo- 
tion, small  though  it  may  be,  than  that  the  motions  of  the 
matter  of  which  it  is  formed  were  so  absolutely  balanced 
as  to  leave  this  nebula  in  the  unique  position  of  absolute 
immobility  in  the  midst  of  whirling  and  drifting  suns  and 
systems  of  suns. 

The  spectroscopic  methods  of  determining  celestial  mo- 
tions in  the  line  of  sight  has  recently  become  fruitful  in  a 
new  but  not  altogether  unforeseen  direction,  for  it  has, 
so  to  speak,  given  us  a  separating  power  far  beyond  that 
of  any  telescope  the  glass-maker  and  the  optician  could 
construct,  and  so  enabled  us  to  penetrate  into  mysteries 


428  HUGGINS 

hidden  in  stars  apparently  single,  and  altogether  unsus- 
pected of  being  binary  systems.  The  spectroscope  has  not 
simply  added  to  the  list  of  the  known  binary  stars,  but  has 
given  to  us  for  the  first  time  a  knowledge  of  a  new  class 
of  stellar  systems,  in  which  the  components  are  in  some 
cases  of  nearly  equal  magnitude  and  in  close  proximity, 
and  are  revolving  with  velocities  greatly  exceeding  the 
planetary  velocities  of  our  system. 

The  K  line  in  the  photographs  of  Mizar,  taken  at  the 
Harvard  College  Observatory,  was  found  to  be  double  at 
intervals  of  fifty-two  days.  The  spectrum  was  therefore  not 
due  to  a  single  source  of  light,  but  to  the  combined  effect 
of  two  stars  moving  periodically  in  opposite  directions  in 
the  line  of  sight.  It  is  obvious  that  if  two  stars  revolve 
round  their  common  centre  of  gravity  in  a  plane  not  per- 
pendicular to  the  line  of  sight,  all  the  lines  in  a  spectrum 
common  to  the  two  stars  will  appear  alternately  single  or 
double. 

In  the  case  of  Mizar  and  the  other  stars  to  be  mentioned, 
the  spectroscopic  observations  are  not  as  yet  extended 
enough  to  furnish  more  than  an  approximate  determination 
of  the  elements  of  their  orbits. 

Mizar  especially,  on  account  of  its  relatively  long  period, 
about  105  days,  needs  further  observations.  The  two  stars 
are  moving  each  with  a  velocity  of  about  50  miles  a  second, 
probably  in  elliptical  orbits,  and  are  about  143,000,000 
miles  apart.  The  stars  of  about  equal  brightness  have  to- 
gether a  mass  about  forty  times  as  great  as  that  of  our  sun. 

A  similar  doubling  of  the  lines  showed  itself  in  the  Har- 
vard photographs  of  fi  Aurigae  at  the  remarkably  close  in- 
terval of  almost  exactly  two  days,  indicating  a  period  of 
revolution  of  about  four  days.  According  to  Vogel's  later 
observations,  each  star  has  a  velocity  of  nearly  70  miles  a 
second,  the  distance  between  the  stars  being  little  more 
than  7,500,000  miles,  and  the  mass  of  the  system  4.7  times 
that  of  the  sun.  The  system  is  approaching  us  at  the  speed 
of  about  1 6  miles  a  second. 

The  telescope  could  never  have  revealed  to  us  double 


CELESTIAL  SPECTROSCOPY 


429 


stars  of  this  order.  In  the  case  of  /9  Aurigse,  combining 
Vogel's  distance  with  Pritchard's  recent  determination  of 
the  star's  parallax,  the  greatest  angular  separation  of  the 
stars  as  seen  from  the  earth  would  be  ^-J-^  part  of  a  second 
of  arc,  and  therefore  very  far  too  small  for  detection  by  the 
largest  telescopes.  If  we  take  the  relation  of  aperture  to 
separating  power  usually  accepted,  an  object-glass  of  about 
80  feet  in  diameter  would  be  needed  to  resolve  this  binary 
star.  The  spectroscope  which  takes  no  note  of  distance 
magnifies,  so  to  speak,  this  minute  angular  separation  4,000 
times;  in  other  words,  the  doubling  of  the  lines,  which  is 
the  phenomenon  that  we  have  to  observe,  amounts  to  the 
easily  measurable  quantity  of  twenty  seconds  of  arc. 

There  were  known,  indeed,  variable  stars  of  short  period, 
which  it  had  been  suggested  might  be  explained  on  the 
hypothesis  of  a  dark  body  revolving  about  a  bright  sun  in 
a  few  days;  but  this  theory  was  met  by  the  objection  that 
no  such  systems  of  closely  revolving  suns  were  known  to 
exist. 

The  Harvard  photographs  of  which  we  have  been  speak- 
ing were  taken  with  a  slitless  form  of  spectroscope,  the 
prisms  being  placed,  as  originally  by  Fraunhofer,  before 
the  object-glass  of  the  telescope.  This  method,  though  it 
possesses  some  advantages,  has  the  serious  drawback  of  not 
permitting  a  direct  comparison  of  the  star's  spectrum  with 
the  terrestrial  spectra.  It  is  obviously  unsuited  to  a  variable 
star  like  Algol,  where  one  star  only  is  bright,  for  in  such  a 
case  there  would  be  no  doubling  of  the  lines,  but  only  a 
small  shift  to  and  fro  in  the  spectrum  of  the  lines  of  the 
bright  star  as  it  moved  in  its  orbit  alternately  toward  and 
from  our  system,  which  would  need  for  its  detection  the 
fiducial  positions  of  terrestrial  lines  compared  directly  with 
them. 

For  such  observations  the  Potsdam  spectrograph  was 
well  adapted.  Professor  Vogel  found  that  the  bright  star 
Algol  did  oscillate  backward  and  forward  in  the  visual  direc- 
tion in  a  period  corresponding  to  the  known  variation  of 
its  light.  The  explanation  which  had  been  suggested  for  the 


430 


HUGGINS 


star's  variability,  that  it  was  partially  eclipsed  at  regular  in- 
tervals of  68.8  hours  by  a  dark  companion  large  enough  to 
cut  off  nearly  five  sixths  of  its  light,  was  therefore  the  true 
one.  The  dark  companion,  no  longer  able  to  hide  itself 
by  its  obscureness,  was  brought  out  into  the  light  of  direct 
observation  by  means  of  its  gravitational  effects. 

Seventeen  hours  before  minimum  Algol  is  receding  at 
the  rate  of  about  24^  miles  a  second,  while  seventeen  hours 
after  minimum  it  is  found  to  be  approaching  with  a  speed 
of  about  28^  miles.  From  these  data,  together  with  those 
of  the  variation  of  its  light,  Vogel  found,  on  the  assumption 
that  both  stars  have  the  same  density,  that  the  companion, 
nearly  as  large  as  the  sun  but  with  about  one  fourth  his 
mass,  revolves  with  a  velocity  of  about  55  miles  a  second. 
The  bright  star,  of  about  twice  the  size  and  mass,  moves 
about  the  common  centre  of  gravity  with  the  speed  of 
about  26  miles  a  second.  The  system  of  the  two  stars, 
which  are  about  3,250,000  miles  apart,  considered  as  a 
whole,  is  approaching  us  with  a  velocity  of  24  miles  a  sec- 
ond. The  great  difference  in  luminosity  of  the  two  stars, 
not  less  than  fifty  times,  suggests  rather  that  they  are  in 
different  stages  of  condensation,  and  dissimilar  in  density. 

It  is  obvious  that  if  the  orbit  of  a  star  with  an  obscure 
companion  is  sufficiently  inclined  to  the  line  of  sight,  the 
companion  will  pass  above  or  below  the  bright  star  and 
produce  no  variation  of  its  light.  Such  systems  may  be 
numerous  in  the  heavens.  In  Vogel's  photographs,  Spica, 
which  is  not  variable,  by  a  small  shifting  of  its  lines  reveals 
a  backward  and  forward  periodical  pulsation  due  to  orbital 
motion.  As  the  pair  whirl  round  their  common  centre  of 
gravity,  the  bright  star  is  sometimes  advancing,  at  others 
receding.  They  revolve  in  about  four  days,  each  star  mov- 
ing with  a  velocity  of  about  56  miles  a  second  in  an  orbit 
probably  nearly  circular,  and  possess  a  combined  mass  of 
rather  more  than  two  and  a  half  times  that  of  the  sun.  Tak- 
ing the  most  probable  value  for  the  star's  parallax,  the 
greatest  angular  separation  of  the  stars  would  be  far  too 
small  to  be  detected  with  the  most  powerful  telescopes. 


CELESTIAL  SPECTROSCOPY  431 

If  in  a  close  double  star  the  fainter  companion  is  of  the 
white-star  type,  while  the  bright  star  is  solar  in  character, 
the  composite  spectrum  would  be  solar  with  the  hydrogen 
lines  unusually  strong.  Such  a  spectrum  would  in  itself 
afford  some  probability  of  a  double  origin,  and  suggest  the 
existence  of  a  companion  star.  In  the  case  of  a  true  binary 
star  the  orbital  motions  of  the  pair  would  reveal  themselves 
in  a  small  periodical  swaying  of  the  hydrogen  lines  rela- 
tively to  the  solar  ones. 

Professor  Pickering  considers  that  his  photographs 
show  ten  stars  with  composite  spectra;  of  these,  five  are 
known  to  be  double.  The  others  are :  r  Persei,  f  Aurigae, 
B  Sagittarii,  31  Ceti,  and  £  Capricorni.  Perhaps  £  Kyrae 
should  be  added  to  this  list. 

In  his  recent  classical  work  on  the  rotation  of  the  sun, 
Duner  has  not  only  determined  the  solar  rotation  for  the 
equator,  but  for  different  parallels  of  latitude  up  to  75°. 
The  close  accordance  of  his  results  shows  that  these  obser- 
vations are  sufficiently  accurate  to  be  discussed  with  the 
variation  of  the  solar  rotation  for  different  latitudes,  which 
had  been  determined  by  the  older  astronomical  methods 
from  the  observations  of  the  solar  spots. 

Though  I  have  already  spoken  incidentally  of  the  in- 
valuable aid  which  is  furnished  by  photography  in  some  of 
the  applications  of  the  spectroscope  to  the  heavenly  bodies, 
the  new  power  which  modern  photography  has  put  into  the 
hands  of  the  astronomer  is  so  great,  and  has  led  already, 
within  the  last  few  years,  to  new  acquisitions  or  knowledge 
of  such  vast  importance  that  it  is  fitting  that  a  few  sen- 
tences should  be  specially  devoted  to  this  subject. 

Photography  is  no  new  discovery,  being  about  half  a 
century  old;  it  may  excite  surprise,  and  indeed  possibly 
suggest  some  apathy  on  the  part  of  astronomers,  that 
though  the  suggestion  of  the  application  of  photography 
to  the  heavenly  bodies  dates  from  the  memorable  occasion 
when,  in  1839,  Arago,  announcing  to  the  Academic  des  Sci- 
ences the  great  discovery  of  Niepce  and  Daguerre,  spoke  of 
the  possibility  of  taking  pictures  of  the  sun  and  moon  by 


432 


HUGGINS 


the  new  process,  yet  that  it  is  only  within  a  few  years  that 
notable  advances  in  astronomical  methods  and  discovery 
have  been  made  by  its  aid. 

The  explanation  is  to  be  found  in  the  comparative  un- 
suitability  of  the  earlier  photographic  methods  for  use  in 
the  observatory.  In  justice  to  the  early  workers  in  astro- 
nomical photography,  among  whom  Bond,  De  la  Rue,  J. 
W.  Draper,  Rutherfurd,  and  Gould  hold  a  foremost  place, 
it  is  needful  to  state  clearly  that  the  recent  great  successes 
in  astronomical  photography  are  not  due  to  greater  skill, 
nor,  to  any  great  extent,  to  superior  instruments,  but  to  the 
very  great  advantages  which  the  modern  gelatine  dry  plate 
possesses  for  use  in  the  observatory  over  the  methods  of 
Daguerre,  and  even  over  the  wet  collodion  film  on  glass 
which,  though  a  great  advance  on  the  silver  plate,  went  but 
a  little  way  toward  putting  into  the  hands  of  the  astronomer 
a  photographic  surface  adapted  fully  to  his  wants. 

The  modern  silver-bromide  gelatine  plate,  except  for  its 
grained  texture,  meets  the  needs  of  the  astronomer  at  all 
points.  It  possesses  extreme  sensitiveness;  it  is  always 
ready  for  use;  it  can  be  placed  in  a^ny  position;  it  can  be 
exposed  for  hours;  lastly,  it  does  not  need  immediate  de- 
velopment, and  for  this  reason  can  be  exposed  again  to  the 
same  object  on  succeeding  nights,  so  as  to  make  up  by 
several  instalments,  as  the  weather  may  permit,  the  total 
time  of  exposure  which  is  deemed  necessary. 

Without  the  assistance  of  photography,  however  greatly 
the  resources  of  genius  might  overcome  the  optical  and 
mechanical  difficulties  of  constructing  large  telescopes,  the 
astronomer  would  have  to  depend  in  the  last  resource  upon 
his  eye.  Now,  we  can  not  by  the  force  of  continued  look- 
ing bring  into  view  an  object  too  feebly  luminous  to  be 
seen  at  the  first  and  keenest  moment  of  vision.  But  the 
feeblest  light  which  falls  upon  the  plate  is  not  lost,  but  it 
is  taken  in  and  stored  up  continuously.  Each  hour  the 
plate  gathers  up  3,600  times  the  light  energy  which  is  re- 
ceived during  the  first  second.  It  is  by  this  power  of  accu- 
mulation that  the  photographic  plate  may  be  said  to  in- 


CELESTIAL  SPECTROSCOPY 


433 


crease,  almost  without  limit,  though  not  in  separating 
power,  the  optical  means  at  the  disposal  of  the  astronomer 
for  the  discovery  or  the  observation  of  faint  objects. 

Two  principal  directions  may  be  pointed  out  in  which 
photography  is  of  great  service  to  the  astronomer.  It  en- 
ables him  within  the  comparatively  short  time  of  a  single 
exposure  to  secure  permanently,  with  great  exactness,  the 
relative  positions  of  hundreds  or  even  of  thousands  of  stars, 
or  the  minute  features  of  nebulae  or  other  objects,  or  the 
phenomena  of  a  passing  eclipse,  tasks  which  by  means  of 
the  eye  and  hand  could  only  be  accomplished,  if  at  all,  after 
a  very  great  expenditure  of  time  and  labour.  Photography 
puts  it  in  the  power  of  the  astronomer  to  accomplish  in  the 
short  span  of  his  own  life,  and  so  enter  into  their  fruition, 
great  works  which  otherwise  must,  have  been  passed  on  by 
him  as  a  heritage  of  labour  to  succeeding  generations. 

The  second  great  service  which  photography  renders  is 
not  simply  an  aid  to  the  powers  the  astronomer  already 
possesses.  On  the  contrary,  the  plate,  by  recording  light- 
waves which  are  both  too  small  and  too  large  to  excite 
vision  in  the  eye,  brings  him  into  new  regions  of  knowl- 
edge, such  as  the  infra-red  and  the  ultra-violet  parts  of  the 
spectrum,  which  must  have  remained  forever  unknown  but 
for  artificial  help. 

The  present  year  will  be  memorable  in  astronomical  his- 
tory for  the  poetical  beginning  of  the  "  Photographic  Chart 
and  Catalogue  of  the  Heavens/'  which  took  their  origin  in 
an  International  Conference  which  met  in  Paris,  in  1887, 
by  the  invitation  of  M.  1'Amiral  Mouchex,  director  of  the 
Paris  Observatory. 

The  richness  in  stars  down  to  the  ninth  magnitude  qf 
the  photographs  of  the  comet  of  1882,  taken  at  the  Cape 
Observatory  under  the  superintendence  of  Dr.  Gill,  and  the 
remarkable  star  charts  of  the  brothers  Henry  which  fol- 
lowed two  years  later,  astonished  the  astronomical  world. 
The  great  excellence  of  these  photographs,  which  was  due 
mainly  to  the  superiority  of  the  gelatine  plate,  suggested  to 
these  astronomers  a  complete  map  of  the  sky,  and  a  little 
28 


434 


HUGGINS 


later  gave  birth  in  the  minds  of  the  Paris  astronomers  to 
the  grand  enterprise  of  an  "  International  Chart  of  the 
Heavens."  The  actual  beginning  of  the  work  this  year  is 
in  no  small  degree  due  to  the  great  energy  and  tact  with 
which  the  director  of  the  Paris  Observatory  has  conducted 
the  initial  steps,  through  the  many  delicate  and  difficult 
questions  which  have  unavoidably  presented  themselves  in 
an  undertaking  which  depends  upon  the  harmonious  work- 
ing in  common  of  many  nationalities,  and  of  no  fewer  than 
eighteen  observatories  in  all  parts  of  the  world.  The  three 
years  since  1887  have  not  been  too  long  for  the  detailed 
organization  of  this  work,  which  has  called  for  several  elab- 
orate preliminary  investigations  on  special  points  in  which 
our  knowledge  was  insufficient,  and  which  have  been  ably 
carried  out  by  Professors  Vogel  and  Bakhuyzen,  Dr.  Tre- 
pied,  Dr.  Scheiner,  Dr.  Gill,  the  Astronomer  Royal,  and 
others.  Time  also  was  required  for  the  construction  of  the 
new  and  special  instruments. 

The  decisions  of  the  conference  in  their  final  form  pro- 
vide for  the  construction  of  a  great  photographic  chart  of 
the  heavens,  with  exposures  corresponding  to  forty  min- 
utes' exposure  at  Paris,  which  it  is  expected  will  reach 
down  to  stars  of  about  the  fourteenth  magnitude.  As  each 
plate  is  to  be  limited  to  four  square  degrees,  and  as  each 
star,  to  avoid  possible  errors,  is  to  appear  on  two  plates, 
over  22,000  photographs  will  be  required.  For  the  more 
accurate  determination  of  the  positions  of  the  stars,  a  reseau 
with  lines  at  distances  of  five  millimetres  apart  is  to  be  pre- 
viously impressed  by  a  faint  light  upon  the  plate,  so  that 
the  image  of  the  reseau  will  appear  together  with  the  images 
of  the  stars  when  the  plate  is  developed.  This  great  work 
will  be  divided,  according  to  their  latitudes,  among  eighteen 
observatories  provided  with  similar  instruments,  though  not 
necessarily  constructed  by  the  same  maker.  Those  in  the 
British  dominions  and  at  Tacubaya  have  been  constructed 
by  Sir  Howard  Grubb. 

Besides  the  plates  to  form  the  great  chart,  a  second  set 
of  plates  for  a  catalogue  is  to  be  taken,  with  a  shorter  ex- 


CELESTIAL  SPECTROSCOPY  435 

posure,  which  will  give  stars  to  the  eleventh  magnitude 
only.  These  plates,  by  a  recent  decision  of  the  permanent 
committee,  are  to  be  pushed  on  as  actively  as  possible, 
though,  as  far  as  may  be  practicable,  plates  for  the  charts 
are  to  be  taken  concurrently.  Photographing  the  plates 
for  the  catalogue  is  but  the  first  step  in  this  work,  and  only 
supplies  the  data  for  the  elaborate  measurements  which 
have  to  be  made,  which  are,  however,  less  laborious  than 
would  be  required  for  a  similar  catalogue  without  the  aid 
of  photography. 

Already  Dr.  Gill  has  nearly  brought  to  conclusion,  with 
the  assistance  of  Professor  Kapteyn,  a  preliminary  photo- 
graphic survey  of  the  southern  heavens. 

With  an  exposure  sufficiently  long  for  the  faintest  stars 
to  impress  themselves  upon  the  plate,  the  accumulating 
action  still  goes  on  for  the  brighter  stars,  producing  a  great 
enlargement  of  their  images  from  optical  and  photographic 
causes.  The  question  has  occupied  the  attention  of  many 
astronomers  whether  it  is  possible  to  find  a  law  connecting 
the  diameters  of  these  more  or  less  over-exposed  images 
with  the  relative  brightness  of  the  stars  themselves.  The 
answer  will  come  out  undoubtedly  in  the  affirmative,  though 
at  present  the  empirical  formulae  which  have  been  sug- 
gested for  this  purpose  differ  from  each  other.  Captain 
Abney  proposes  to  measure  the  total  photographic  action, 
including  density  as  well  as  size,  by  the  obstruction  which 
the  stellar  image  offers  to  light. 

A  further  question  follows  as  to  the  relation  which  the 
photographic  magnitudes  of  stars  bears  to  those  determined 
by  eye.  Visual  magnitudes  are  the  physiological  expres- 
sion of  the  eye's  integration  of  that  part  of  the  star's  light 
which  extends  from  the  red  to  the  blue.  Photographic 
magnitudes  represent  the  plate's  integration  of  another  part 
of  the  star's  light — namely,  from  a  little  below  where  the 
power  of  the  eye  leaves  off  in  the  blue  to  where  the  light 
is  cut  off  by  the  glass,  or  is  greatly  reduced  by  want  of 
proper  corrections  when  a  refracting  telescope  is  used.  It 
is  obvious  that  the  two  records  are  taken  by  different  meth- 


436  HUGGINS 

ods  in  dissimilar  units  of  different  parts  of  the  star's  light. 
In  the  case  of  certain  coloured  stars  the  photographic 
brightness  is  very  different  from  the  visual  brightness;  but 
in  all  stars  changes,  especially  of  a  temporary  character, 
may  occur  in  the  photographic  or  the  visual  region,  unac- 
companied by  similar  changes  in  the  other  part  of  the 
spectrum.  For  these  reasons  it  would  seem  desirable  that 
the  two  sets  of  magnitudes  should  be  tabulated  independ- 
ently, and  be  regarded  as  supplementary  of  each  other. 

The  determination  of  the  distances  of  the  fixed  stars 
from  the  small  apparent  shift  of  their  positions  when  viewed 
from  widely  separated  positions  of  the  earth  in  its  orbit  is 
one  of  the  most  refined  operations  of  the  observatory.  The 
great  precision  with  which  this  minute  angular  quantity,  a 
fraction  of  a  second  of  arc  only,  has  to  be  measured,  is  so 
delicate  an  operation  with  the  ordinary  micrometer — 
though,  indeed,  it  was  with  this  instrument  that  the  classi- 
cal observations  of  Sir  Robert  Ball  were  made — that  a  spe- 
cial instrument,  in  which  the  measures  are  made  by  moving 
the  two  halves  of  a  divided  object-glass,  known  as  a  heli- 
ometer,  has  been  pressed  into  this  service,  and  quite  re- 
cently, in  the  skilful  hands  of  Dr.  Gill  and  Dr.  Elkin,  has 
largely  increased  our  knowledge  in  this  direction. 

It  is  obvious  that  photography  might  be  here  of  good 
service,  if  we  could  rely  upon  measurements  of  photo- 
graphs of  the  same  stars  taken  at  suitable  intervals  of  time. 
Professor  Pritchard,  to  whom  is  due  the  honour  of  having 
opened  this  new  path,  aided  by  his  assistants,  has  proved 
by  elaborate  investigations  that  measures  for  parallax  may 
be  safely  made  upon  photographic  plates,  with,  of  course, 
the  advantages  of  leisure  and  repetition ;  and  he  has  already 
by  this  method  determined  the  parallax  for  twenty-one 
stars  with  an  accuracy  not  inferior  to  that  of  values  pre- 
viously obtained  by  purely  astronomical  methods. 

The  remarkable  successes  of  astronomical  photography, 
which  depend  upon  the  plate's  power  of  accumulation  of  a 
very  feeble  light  acting  continuously  through  an  exposure 
of  several  hours,  are  worthy  to  be  regarded  as  a  new  revela- 


CELESTIAL  SPECTROSCOPY  437 

tion.  The  first  chapter  opened  when,  in  1880,  Dr.  Henry 
Draper  obtained  a  picture  of  the  nebula  of  Orion;  but  a 
more  important  advance  was  made  in  1883,  when  Dr.  Com- 
mon, by  his  photographs,  brought  to  our  knowledge  details 
and  extensions  of  this  nebula  hitherto  unknown.  A  further 
disclosure  took  place  in  1885,  when  the  brothers  Henry 
showed  for  the  first  time  in  great  detail  the  spiral  nebulosity 
issuing  from  the  bright  star  Maia  of  the  Pleiades,  and 
shortly  afterward  nebulous  streams  about  the  other  stars 
of  this  group.  In  1886  Mr.  Roberts,  by  means  of  a  photo- 
graph to  which  three  hours'  exposure  had  been  given, 
showed  the  whole  background  of  this  group  to  be  nebulous. 
In  the  following  year  Mr.  Roberts  more  than  doubled  for 
us  the  great  extension  of  the  nebular  region  which  sur- 
rounds the  trapezium  in  the  constellation  of  Orion.  By  his 
photographs  of  the  great  nebula  in  Andromeda  he  has 
shown  the  true  significance  of  the  dark  canals  which  had 
been  seen  by  the  eye.  They  are  in  reality  spaces  between 
successive  rings  of  bright  matter,  which  appeared  nearly 
straight,  owing  to  the  inclination  in  which  they  lie  relatively 
to  us.  These  bright  rings  surround  an  undefined  central 
luminous  mass.  I  have  already  spoken  of  this  photograph. 

Some  recent  photographs  by  Mr.  Russell  show  that  the 
great  rift  in  the  Milky  Way  in  Argus,  which  to  the  eye  is 
void  of  stars,  is  in  reality  uniformly  covered  with  them. 
Also  quite  recently  Mr.  George  Hale  has  photographed  the 
solar  prominences  by  means  of  a  grating,  making  use  of  the 
lines  H  and  K. 

The  heavens  are  richly  but  very  irregularly  inwrought 
with  stars.  The  brighter  stars  cluster  into  well-known 
groups  upon  a  background  formed  of  an  enlacement  of 
streams  and  convoluted  windings  and  intertwined  spirals 
of  fainter  stars,  which  becomes  richer  and  more  intricate 
in  the  irregularly  rifted  zone  of  the  Milky  Way. 

We,  who  form  part  of  the  emblazonry,  can  only  see  the 
design  distorted  and  confused;  here  crowded,  there  scat- 
tered, at  another  place  superposed.  The  groupings  due  to 
our  position  are  mixed  up  with  those  which  are  real. 


438  MUGGINS 

Can  we  suppose  that  each  luminous  point  has  no  other 
relation  to  those  near  it  than  the  accidental  neighbourship 
of  grains  of  sand  upon  the  shore,  or  of  particles  of  the  wind- 
blown dust  of  the  desert?  Surely  every  star  from  Sirius 
and  Vega  down  to  each  grain  of  the  light-dust  of  the  Milky 
Way  has  its  present  place  in  the  heavenly  pattern  from  the 
slow  evolving  of  its  past.  We  see  a  system  of  systems,  for 
the  broad  features  of  clusters  and  streams  and  spiral  wind- 
ings which  mark  the  general  design  are  reproduced  in  every 
part.  The  whole  is  in  motion,  each  point  shifting  its  posi- 
tion by  miles  every  second,  though  from  the  august  magni- 
tude of  their  distances  from  us  and  from  each  other  it  is 
only  by  the  accumulated  movements  of  years  or  of  genera- 
tions that  some  small  changes  of  relative  position  reveal 
themselves. 

The  deciphering  of  this  wonderfully  intricate  constitu- 
tion of  the  heavens  will  be  undoubtedly  one  of  the  chief 
astronomical  works  of  the  coming  century.  The  primary 
task  of  the  sun's  motion  in  space,  together  with  the 
motions  of  the  brighter  stars,  has  been  already  put  well 
within  our  reach  by  the  spectroscopic  method  of  the  meas- 
urement of  star  motions  in  the  line  of  sight. 

From  other  directions  information  is  accumulating: 
from  photographs  of  clusters  and  parts  of  the  Milky  Way, 
by  Roberts  in  this  country,  Barnard  at  the  Lick  Observa- 
tory, and  Russell  at  Sydney;  from  the  counting  of  stars,  and 
the  detection  of  their  configurations,  by  Holden  and  by 
Backhouse;  from  the  mapping  of  the  Milky  Way  by  eye, 
at  Parsonstown;  from  photographs  of  the  spectra  of  stars, 
by  Pickering  at  Harvard  and  in  Peru;  and  from  the  exact 
portraiture  of  the  heavens  in  the  great  international  star 
chart  which  begins  this  year. 

I  have  but  touched  some  only  of  the  problems  of  the 
newer  side  of  astronomy.  Of  the  many  others  which  would 
claim  our  attention  if  time  permitted  I  may  name  the  fol- 
lowing: The  researches  of  the  Earl  of  Rosse  on  lunar  radia- 
tion, and  the  work  on  the  same  subject  and  on  the  sun  by 
Langley;  observations  of  lunar  heat  with  an  instrument 


CELESTIAL  SPECTROSCOPY  439 

of  his  own  invention  by  Mr.  Boys,  and  observations  of 
the  variation  of  the  moon's  heat  with  its  phase  by  Mr. 
Frank  Very;  the  discovery  of  the  ultra-violet  part  of  the 
hydrogen  spectrum,  not  in  the  laboratory,  but  from  the 
stars;  the  confirmation  of  this  spectrum  by  terrestrial  hy- 
drogen in  part  by  W.  H.  Vogel,  and  in  its  all  but  complete 
form  by  Cornu,  who  found  similar  series  in  the  ultra-violet 
spectra  of  aluminium  and  thallium;  the  discovery  of  a 
simple  formula  for  the  hydrogen  series  by  Balmer;  the 
important  question  as  to  the  numerical  spectral  relationship 
of  different  substances,  especially  in  connection  with  their 
chemical  properties,  and  the  further  question  as  to  the 
origin  of  the  harmonic  and  other  relations  between  the  lines 
and  the  groupings  of  lines  of  spectra.  On  these  points  con- 
tributions during  the  past  year  have  been  made  by  Rudolf 
von  Kovesligethy,  Ames,  Hartley,  Deslandres,  Rydberg, 
Grunwald,  Kayser  and  Runge,  Johnstone  Stoney,  and 
others;  the  remarkable  employment  of  interference  phe- 
nomena by  Professor  Michelson  for  the  determination  of 
the  size,  and  distribution  of  light  within  them,  of  the  images 
of  objects  which,  when  viewed  in  a  telescope,  subtend  an 
angle  less  than  that  subtended  by  the  light-wave  at  a  dis- 
tance equal  to  the  diameter  of  the  objective;  a  method 
applicable  not  alone  to  celestial  objects,  but  also  to  spectral 
lines,  and  other  questions  of  molecular  physics. 

Along  the  other  lines  there  has  not  been  less  activity; 
by  newer  methods,  by  the  aid  of  larger  or  more  accurately 
constructed  instruments,  by  greater  refinement  of  analysis, 
knowledge  has  been  increased,  especially  in  precision  and 
minute  exactness. 

Astronomy,  the  oldest  of  the  sciences,  has  more  than 
renewed  her  youth.  At  no  time  in  the  past  has  she  been 
so  bright  with  unbounded  aspirations  and  hopes.  Never 
were  her  temples  so  numerous,  nor  the  crowd  of  her 
votaries  so  great.  The  British  Astronomical  Association, 
formed  within  the  year,  numbers  already  about  six  hundred 
members.  Happy  is  the  lot  of  those  who  are  still  on  the 
eastern  side  of  life's  meridian! 


440  HUGGINS 

Already,  alas!  the  original  founders  of  the  newer  meth- 
ods are  falling  out — Kirchhoff,  Angstrom,  D' Arrest,  Sec- 
chi,  Draper,  Becquerel;  but  their  places  are  more  than 
filled;  the  pace  of  the  race  is  gaining,  but  the  goal  is  not 
and  never  will  be  in  sight. 

Since  the  time  of  Newton  our  knowledge  of  the  phe- 
nomena of  Nature  has  wonderfully  increased,  but  man  asks, 
perhaps  more  earnestly  now  than  then,  What  is  the  ultimate 
reality  behind  the  reality  of  the  perceptions?  Are  they  only 
the  pebbles  of  the  beach  with  which  we  have  been  playing? 
Does  not  the  ocean  of  ultimate  reality  and  truth  lie  beyond? 


THE    NEW  ASTRONOMY: 
A    PERSONAL    RETROSPECT1 

WHILE  progress  in  all  branches  of  knowledge  has 
been  rapid  beyond  precedent  during  the  past  sixty 
years,  in  at  least  two  directions  this  knowledge 
has  been  so  unexpected  and  novel  in  character  that  two  new 
sciences  may  be  said  to  have  arisen :  the  new  medicine,  with 
which  the  names  of  Lister  and  of  Pasteur  will  remain  asso- 
ciated; and  the  new  astronomy,  of  the  birth  and  early 
growth  of  which  I  have  now  to  speak. 

The  New  Astronomy,  unlike  the  old  astronomy  to 
which  we  are  indebted  for  skill  in  the  navigation  of  the 
seas,  the  calculation  of  the  tides,  and  the  daily  regulation 
of  time,  can  lay  no  claim  to  afford  us  material  help  in  the 
routine  of  daily  life.  Her  sphere  lies  outside  the  earth. 
Is  she  less  fair?  Shall  we  pay  her  less  court  because  it  is 
to  mental  culture  in  its  highest  form,  to  our  purely  intel- 
lectual joys,  that  she  contributes?  For  surely  in  no  part 
of  Nature  are  the  noblest  and  most  profound  conceptions  of 
the  human  spirit  more  directly  called  forth  than  in  the  study 
of  the  heavens  and  the  host  thereof. 

"  That  with  the  glorie  of  so  goodly  sight 
The  hearts  of  men  .  .  . 
.  .  .  may  lift  themselves  up  hyer." 

May  we  not  rather  greet  her  in  the  words  of  Horace:  "  O 
matre  pulchra  filia  pulchrior  "? 

As  it  fell  to  my  lot  to  have  some  part  in  the  early  de- 
velopment of  this  new  science,  it  has  been  suggested  to  me 
1  From  the  "  Nineteenth  Century,"  June,  1897. 
44* 


442  HUGGINS 

that  the  present  Jubilee  year  of  retrospect  would  be  a  suit- 
able occasion  to  give  some  account  of  its  history  from  the 
standpoint  of  my  own  work. 

Before  I  begin  the  narrative  of  my  personal  observa- 
tions it  is  desirable  that  I  should  give  a  short  statement 
of  the  circumstances  which  led  up  to  the  birth  of  the  new 
science  in  1859,  and  also  say  a  few  words  of  the  state  of 
scientific  opinion  about  the  matters  of  which  it  treats,  just 
before  that  time. 

It  is  not  easy  for  men  of  the  present  generation,  familiar 
with  the  knowledge  which  the  new  methods  of  research 
of  which  I  am  about  to  speak  have  revealed  to  us,  to  put 
themselves  back  a  generation,  into  the  position  of  the  sci- 
entific thought  which  existed  on  these  subjects  in  the  early 
years  of  the  Queen's  reign.  At  that  time  any  knowledge 
of  the  chemical  nature  and  of  the  physics  of  the  heavenly 
bodies  was  regarded  as  not  only  impossible  of  attainment 
by  any  methods  of  direct  observation,  but  as,  indeed,  lying 
altogether  outside  the  limitations  imposed  upon  man  by 
his  senses,  and  by  the  fixity  of  his  position  upon  the  earth. 

It  could  never  be,  it  was  confidently  thought,  more  than 
a  matter  of  presumption,  whether  even  the  matter  of  the 
sun,  and  much  less  that  of  the  stars,  were  of  the  same  nature 
as  that  of  the  earth,  and  the  unceasing  energy  radiated  from 
it  due  to  such  matter  at  a  high  temperature.  The  nebular 
hypothesis  of  Laplace  at  the  end  of  the  last  century  re- 
quired, indeed,  that  matter  similar  to  that  of  the  earth 
should  exist  throughout  the  solar  system;  but  then  this 
hypothesis  itself  needed  for  its  full  confirmation  the  inde- 
pendent and  direct  observation  that  the  solar  matter  was 
terrestrial  in  its  nature.  This  theoretical  probability  in  the 
case  of  the  sun  vanished  almost  into  thin  air  when  the 
attempt  was  made  to  extend  it  to  the  stellar  hosts;  for  it 
might  well  be  urged  that  in  those  immensely  distant  regions 
an  original  difference  of  the  primordial  stuff  as  well  as  other 
conditions  of  condensation  were  present,  giving  rise  to 
groups  of  substances  which  have  but  little  analogy  with 
those  of  our  earthly  chemistry. 


THE   NEW  ASTRONOMY  443 

About  the  time  of  the  Queen's  accession  to  the  throne 
the  French  philosopher  Comte  put  very  clearly  in  his 
"  Cours  de  Philosophic  Positive  "  the  views  then  held  of 
the  impossibility  of  direct  observations  of  the  chemical  na- 
ture of  the  heavenly  bodies.  He  says: 

"  On  congoit  en  effet,  que  nous  puissions  conjecturer, 
avec  quelque  espoir  de  succes,  sur  la  formation  du  systeme 
solaire  dont  nous  faisons  partie,  car  il  nous  presente  de 
nombreux  phenomenes  parfaitement  connus,  susceptibles 
peut-etre  de  porter  un  temoignage  decisif  de  sa  veritable 
origine  immediate.  Mais  quelle  pourrait  etre,  au  contraire, 
la  base  rationnelle  de  nos  conjectures  sur  la  formation  des 
soleils  eux-memes?  Comment  confirmer  ou  infirmer  a  ce 
sujet,  d'apres  les  phenomenes,  aucune  hypothese  cosmo- 
gonique,  lorsqu'il  n'existe  vraiment  en  ce  genre  aucun 
phenomene  explore,  ni  meme,  sans  doute,  EXPLOR- 
ABLE?  "  (The  capitals  are  mine.) 

We  could  never  know  for  certain,  it  seemed,  whether 
the  matter  and  the  forces  with  which  we  are  familiar  are 
peculiar  to  the  earth,  or  are  common  with  it  to  the  mid- 
night sky, 

"  All  sow'd  with  glistering  stars  more  thicke  than  grasse, 
Whereof  each  other  doth  in  brightnesse  passe." 

For  how  could  we  extend  the  methods  of  the  laboratory  to 
bodies  at  distances  so  great  that  even  the  imagination  fails 
to  realize  them? 

The  only  communication  from  them  which  reaches  us 
across  the  gulf  of  space  is  the  light  which  tells  us  of  their 
existence.  Fortunately  this  light  is  not  so  simple  in  its 
nature  as  it  seems  to  be  to  the  unaided  eye.  In  reality  it 
is  very  complex;  like  a  cable  of  many  strands,  it  is  made 
up  of  light-rays  of  many  kinds.  Let  this  light-cable  pass 
from  air  obliquely  through  a  piece  of  glass,  and  its  sepa- 
rate strand-rays  all  go  astray,  each  turning  its  own  way, 
and  then  go  on  apart.  Make  the  glass  into  the  shape  of 
a  wedge  or  prism,  and  the  rays  are  twice  widely  scat- 
tered. 


444  HUGGINS 

"First  the  flaming  red 

Sprung  vivid  forth:  the  tawny  orange  next; 
And  next  delicious  yellow;  by  whose  side 
Fell  the  kind  beams  of  all-refreshing  green. 
Then  the  pure  blue,  that  swells  autumnal  skies, 
Ethereal  played;  and  then,  of  sadder  hue, 
Emerged  the  deepened  indigo,  as  when 
The  heavy-skirted  evening  droops  with  frost; 
While  the  last  gleamings  of  refracted  light 
Died  in  the  fainting  violet  away." 

Within  this  unravelled  starlight  exists  a  strange  cryptog- 
raphy. Some  of  the  rays  may  be  blotted  out,  others  may 
be  enhanced  in  brilliancy.  These  differences,  countless  in 
variety,  form  a  code  of  signals,  in  which  is  conveyed  to  us, 
when  once  we  have  made  out  the  cipher  in  which  it  is 
written,  information  of  the  chemical  nature  of  the  celestial 
gases  by  which  the  different  light-rays  have  been  blotted 
out,  or  by  which  they  have  been  enhanced.  In  the  hands 
of  the  astronomer  a  prism  has  now  become  more  potent  in 
revealing  the  unknown  than  even  was  said  to  be  "  Agrippa's 
magic  glass." 

It  was  the  discovery  of  this  code  of  signals,  and  of  its 
interpretation,  which  made  possible  the  rise  of  the  new 
astronomy.  We  must  glance,  but  very  briefly,  at  some  of 
the  chief  steps  in  the  progress  of  events  which  slowly  led 
up  to  this  discovery. 

Newton,  in  his  classical  work  upon  the  solar  spectrum, 
failed  through  some  strange  fatality  to  discover  the  nar- 
row gaps  wanting  in  light,  which,  as  dark  lines,  cross  the 
colours  of  the  spectrum  and  constitute  the  code  of  symbols. 
His  failure  is  often  put  down  to  his  using  a  round  hole  in 
place  of  a  narrow  slit,  through  the  overlapping  of  the 
images  of  which  the  dark  lines  failed  to  show  themselves. 
Though  Newton  did  use  a  round  hole,  he  states  distinctly 
in  his  "  Optics  "  that  later  he  adopted  a  narrow  opening  in 
the  form  of  a  long  parallelogram — that  is,  a  true  slit — at 
first  one  tenth  of  an  inch  in  width,  then  only  one  twentieth 
of  an  inch,  and  at  last  still  narrower.  These  conditions  un- 
der which  Newton  worked  were  such  as  should  have  shown 
him  the  dark  lines  upon  his  screen.  Professor  Johnson  has 


THE   NEW  ASTRONOMY  445 

recently  repeated  Newton's  experiments  under  strictly 
similar  conditions,  with  the  result  that  the  chief  dark  lines 
were  well  seen.  For  some  reason  Newton  failed  to  discover 
them.  A  possible  cause  may  have  been  the  bad  annealing 
of  his  prism,  though  he  says  that  it  was  made  of  good  glass 
and  free  from  bubbles. 

The  dark  lines  were  described  first  by  Wollaston  in 
1792,  who  strangely  associated  them  with  the  boundaries 
of  the  spectral  colours,  and  so  turned  contemporary  thought 
away  from  the  direction  in  which  lay  their  true  significance. 
It  was  left  to  Fraunhofer,  in  1815,  by  whose  name  the  dark 
lines  are  still  known,  not  only  to  map  some  six  hundred  of 
them,  but  also  to  discover  similar  lines,  but  differently  ar- 
ranged, in  several  stars.  Further,  he  found  that  a  pair  of 
dark  lines  in  the  solar  spectrum  appeared  to  correspond  in 
their  position  in  the  spectrum,  and  in  their  distance  from 
each  other,  to  a  pair  of  bright  lines  which  were  nearly 
always  present  in  terrestrial  flames.  This  last  observation 
contained  the  key  to  the  interpretation  of  the  dark  lines 
as  a  code  of  symbols;  but  Fraunhofer  failed  to  use  it, 
and  the  birth  of  astrophysics  was  delayed.  An  observation 
by  Forbes  at  the  eclipse  of  1836  led  thought  away  from  the 
suggestive  experiments  of  Fraunhofer,  so  that  in  the  very 
year  of  the  Queen's  accession  the  knowledge  of  the  time 
had  to  be  summed  up  by  Mrs.  Somerville  in  the  negation, 
"  We  are  still  ignorant  of  the  cause  of  these  rayless  bands." 

Later  on,  the  revelation  came  more  oj  less  fully  to  many 
minds.  Foucault,  Balfour  Stewart,  and  Angstrom  prepared 
the  way.  Prophetic  guesses  were  made  by  Stokes  and  by 
Lord  Kelvin.  But  it  was  KirchhofT  who,  in  1859,  first  fully 
developed  the  true  significance  of  the  dark  lines;  and  by  his 
joint  work  with  Bunsen  on  the  solar  spectrum  proved  be- 
yond all  question  that  the  dark  lines  in  the  spectrum  of  the 
sun  are  produced  by  the  absorption  of  the  vapours  of  the 
same  substances,  which  when  suitably  heated  give  out  cor- 
responding bright  lines;  and,  further,  that  many  of  the 
solar  absorbing  vapours  are  those  of  substances  found  upon 
the  earth.  The  new  astronomy  was  born. 


446  HUGGINS 

At  the  time  that  I  purchased  my  present  house,  Tulse 
Hill  was  much  more  than  now  in  the  country  and  away 
from  the  smoke  of  London.  It  was  after  a  little  hesitation 
that  I  decided  to  give  my  chief  attention  to  observational 
astronomy,  for  I  was  strongly  under  the  spell  of  the  rapid 
discoveries  then  taking  place  in  microscopical  research  in 
connection  with  physiology. 

In  1856  I  built  a  convenient  observatory  opening  by  a 
passage  from  the  house,  and  raised  so  as  to  command  an 
uninterrupted  view  of  the  sky  except  on  the  north  side.  It 
consisted  of  a  dome  twelve  feet  in  diameter,  and  a  transit 
room.  There  was  erected  in  it  an  equatorially  mounted 
telescope  by  Dollond  of  five  inches  aperture,  at  that  time 
looked  upon  as  a  large  rather  than  a  small  instrument.  I 
commenced  work  on  the  usual  lines,  taking  transits,  ob- 
serving and  making  drawings  of  planets.  Some  of  Jupiter 
now  lying  before  me,  I  venture  to  think,  would  not  com- 
pare unfavourably  with  drawings  made  with  the  larger  in- 
struments of  the  present  day. 

About  that  time  Mr.  Alvan  Clark,  the  founder  of  the 
American  firm  famous  for  the  construction  of  the  great 
object-glasses  of  the  Lick  and  the  Yerkes  Observatories, 
then  a  portrait  painter  by  profession,  began,  as  an  amateur, 
to  make  object-glasses  of  large  size  for  that  time,  and  of 
very  great  merit.  Specimens  of  his  earliest  work  came  into 
the  hands  of  my  friend  Mr.  Dawes  and  received  the  high 
approval  of  that  distinguished  judge.  In  1858  I  purchased 
from  Mr.  Dawes  an  object-glass  by  Alvan  Clark  of  eight 
inches  diameter,  which  he  parted  with  to  make  room  for  a 
lens  of  a  larger  diameter  by  a  quarter  of  an  inch,  which  Mr. 
Clark  had  undertaken  to  make  for  him.  I  paid  the  price 
that  it  had  cost  Mr.  Dawes — namely,  £200.  This  telescope 
was  mounted  for  me  equatorially  and  provided  with  a  clock 
motion  by  Mr.  Cooke,  of  York. 

I  soon  became  a  little  dissatisfied  with  the  routine  char- 
acter of  ordinary  astronomical  work,  and  in  a  vague  way 
sought  about  in  my  mind  for  the  possibility  of  research 
upon  the  heavens  in  a  new  direction  or  by  new  methods. 


Photogravur 


'immer. 


446  MS 

At  the  time  that  1  ny  present  house,  Tulse 

Hill  was  much  mo;  the  country  and  away 

from  the  smokt  is  after  a  little  hesitation 

that  I  decided  to  ^  ention  to  observational 

astronomy.  :  iie  spell  of  the  rapid 

scopical  research  in 
coniH 

In  ry  opening  by  a 

• 

"•th  side.    It 
i  transit 

.it  time 

ug  before  me,  ^Vse-wow^Jio^-iTOiik)  wou 

unfavourabLyT7with  drawings  .made  with  the  lacger  in- 

jammrW  biBrt-jiX  va  gjfffnusq  B  mofi  siuvBigorolff3 

.ents  oftne  present  day. 

e  Mr.  Alvan  Clark,  the  founder  of  the 

for  the  construction  of  the  great 

Yerkes  Observatories, 


k  came  into 

d  the  high 

ti  1858  I  purchased 

!van  Clark  of  eight 

i;ike  room  for  a 

mch,  which  Mr. 

n.    I  paid  the  price 

-'OO.    This  telescope 

<d  provided  with  a  clock 

. 

isfied  with  the  routine  char- 

actc  Hnary  astronomical  work,  and  in  a  vague  way 

ut  in  my  mind  for  the  possibility  of  research 

ii  a  new  direction  or  by  new  methods. 


THE   NEW   ASTRONOMY  447 

It  was  just  at  this  time,  when  a  vague  longing  after  newer 
methods  of  observation  for  attacking  many  of  the  problems 
of  the  heavenly  bodies  rilled  my  mind,  that  the  news  reached 
me  of  KirchhofFs  great  discovery  of  the  true  nature  and 
the  chemical  constitution  of  the  sun  from  his  interpretation 
of  the  Fraunhofer  lines. 

This  news  was  to  me  like  the  coming  upon  a  spring  of 
water  in  a  dry  and  thirsty  land.  Here  at  last  presented  itself 
the  very  order  of  work  for  which  in  an  indefinite  way  I 
was  looking — namely,  to  extend  his  novel  methods  of  re- 
search upon  the  sun  to  the  other  heavenly  bodies.  A  feel- 
ing as  of  inspiration  seized  me:  I  felt  as  if  I  had  it  now  in 
my  power  to  lift  a  veil  which  had  never  before  been  lifted; 
as  if  a  key  had  been  put  into  my  hands  which  would  unlock 
a  door  which  had  been  regarded  as  forever  closed  to  man — 
the  veil  and  door  behind  which  lay  the  unknown  mystery  of 
the  true  nature  of  the  heavenly  bodies.  This  was  especially 
work  for  which  I  was  to  a  great  extent  prepared,  from  be- 
ing already  familiar  with  the  chief  methods  of  chemical  and 
physical  research. 

It  was  just  at  this  time  that  I  happened  to  meet  at  a 
soiree  of  the  Pharmaceutical  Society,  where  spectroscopes 
were  shown,  my  friend  and  neighbour,  Dr.  W.  Allen  Miller, 
Professor  of  Chemistry  at  King's  College,  who  had  already 
worked  much  on  chemical  spectroscopy.  A  sudden  im- 
pulse seized  me  to  suggest  to  him  that  we  should  return 
home  together.  On  our  way  home  I  told  him  of  what  was 
in  my  mind,  and  asked  him  to  join  me  in  the  attempt  I  was 
about  to  make,  to  apply  Kirchhoff's  methods  to  the  stars. 
At  first,  from  considerations  of  the  great  relative  faint- 
ness  of  the  stars,  and  the  great  delicacy  of  the  work  from 
the  earth's  motion,  even  with  the  aid  of  a  clockwork,  he 
hesitated  as  to  the  probability  of  our  success.  Finally  he 
agreed  to  come  to  my  observatory  on  the  first  fine  evening, 
for  some  preliminary  experiments  as  to  what  we  might  ex- 
pect to  do  upon  the  stars. 

At  that  time  a  star  spectroscope  was  an  instrument  un- 
known to  the  optician.  I  remember  that  for  our  first  trials 


448  HUGGINS 

we  had  one  of  the  hollow  prisms  filled  with  bisulphide  of 
carbon,  so  much  in  use  then,  and  which,  in  consequence  of 
a  small  leak,  smelled  abominably.  To  this  day  this  pungent 
odour  reminds  me  of  star  spectra! 

Let  us  look  at  the  problem  which  lay  before  us.  It  is 
difficult  for  any  one,  who  has  now  only  to  give  an  order  for 
a  star  spectroscope,  to  understand  in  any  true  degree  the 
difficulties  which  we  met  with  in  attempting  to  make  such 
observations  for  the  first  time.  From  the  sun,  with  which 
the  Heidelberg  professors  had  to  do — which,  even  bright 
as  it  is,  for  some  parts  of  the  spectrum  has  no  light  to  spare 
— to  the  brightest  stars  is  a  very  far  cry.  The  light  re- 
ceived at  the  earth  from  a  first-magnitude  star,  as  Vega,  is 
only  about  the  aooooiooocur  Par^  °f  tnat  received  from 
the  sun. 

Fortunately,  as  the  stars  are  too  far  off  to  show  a  true 
disk,  it  is  possible  to  concentrate  all  the  light  received  from 
the  star  upon  a  large  mirror  or  object-glass,  into  the  tele- 
scopic image,  and  so  increase  its  brightness. 

We  could  not  make  use  of  the  easy  method  adopted  by 
Fraunhofer  of  placing  a  prism  before  the  object-glass,  for 
we  needed  a  terrestrial  spectrum,  taken  under  the  same 
conditions,  for  the  interpretation,  by  a  simultaneous  com- 
parison with  it  of  the  star's  spectrum.  Kirchhoff's  method 
required  that  the  image  of  a  star  should  be  thrown  upon  a 
narrow  slit  simultaneously  with  the  light  from  a  flame  or 
from  an  electric  spark. 

These  conditions  made  it  necessary  to  attach  a  spectro- 
scope to  the  eye  end  of  the  telescope,  so  that  it  would  be 
carried  with  it,  with  its  slit  in  the  focal  plane.  Then,  by 
means  of  a  small  reflecting  prism  placed  before  one  half 
of  the  slit,  light  from  a  terrestrial  source  at  the  side  of  the 
telescope  could  be  sent  into  the  instrument  together  with 
the  star's  light,  and  so  form  a  spectrum  by  the  side  of  the 
stellar  spectrum,  for  convenient  comparison  with  it. 

This  was  not  all.  As  the  telescopic  image  of  a  star  is  a 
point,  its  spectrum  will  be  a  narrow  line  of  light  without 
appreciable  breadth.  Now,  for  the  observation  of  either 


THE   NEW   ASTRONOMY 


449 


dark  or  of  bright  lines  across  the  spectrum  a  certain  breadth 
is  absolutely  needful.  To  get  breadth,  the  pointlike  image 
of  the  star  must  be  broadened  out.  As  light  is  of  first 
importance,  it  was  desirable  to  broaden  the  star's  image 
only  in  the  one  direction  necessary  to  give  breadth  to  the 
spectrum;  or,  in  other  words,  to  convert  the  stellar  point 
into  a  short  line  of  light.  Such  an  enlargement  in  one  direc- 
tion only  could  be  given  by  the  device,  first  employed  by 
Fraunhofer  himself,  of  a  lens  convex  or  concave  in  one 
direction  only,  and  flat,  and  so  having  no  action  on  the 
light,  in  a  direction  at  right  angles  to  the  former  one. 

When  I  went  to  the  distinguished  optician,  Mr.  Andrew 
Ross,  to  ask  for  such  a  lens,  he  told  me  that  no  such  lenses 
were  made  in  England,  but  that  the  spectacle  lenses  then 
very  occasionally  required  to  correct  astigmatism — first 
used,  I  believe,  by  the  then  Astronomer  Royal,  the  late  Sir 
George  Airy — were  ground  in  Berlin.  He  procured  for 
me  from  Germany  several  lenses;  but  not  long  afterward  a 
cylindrical  lens  was  ground  for  me  by  Browning.  By  means 
of  such  a  lens,  placed  within  the  focus  of  the  telescope,  in 
front  of  the  slit,  the  pointlike  image  of  a  star  could  be 
widened  in  one  direction  so  as  to  become  a  very  fine  line 
of  light,  just  so  long  as,  but  no  longer  than,  was  necessary 
to  give  to  the  spectrum  a  breadth  sufficient  for  distinguish- 
ing any  lines  by  which  it  may  be  crossed. 

It  is  scarcely  possible  at  the  present  day,  when  all  these 
points  are  as  familiar  as  household  words,  for  any  astron- 
omer to  realize  the  large  amount  of  time  and  labour  which 
had  to  be  devoted  to  the  successful  construction  of  the 
first  star  spectroscope.  Especially  was  it  difficult  to  pro- 
vide for  the  satisfactory  introduction  of  the  light  for  the 
comparison  spectrum.  We  soon  found,  to  our  dismay, 
how  easily  the  comparison  lines  might  become  instrumen- 
tally  shifted,  and  so  be  no  longer  strictly  fiducial.  As  a 
test  we  used  the  solar  lines  as  reflected  to  us  from  the  moon 
— a  test  of  more  than  sufficient  delicacy  with  the  resolving 
power  at  our  command. 

Then  it  was  that  an  astronomical  observatory  began, 
29 


450 


HUGGINS 


for  the  first  time,  to  take  on  the  appearance  of  a  laboratory. 
Primary  batteries,  giving  forth  noxious  gases,  were  ar- 
ranged outside  one  of  the  windows;  a  large  induction  coil 
stood  mounted  on  a  stand  on  wheels  so  as  to  follow  the 
positions  of  the  eye  end  of  the  telescope,  together  with  a 
battery  of  several  Leyden  jars;  shelves  with  Bunsen 
burners,  vacuum  tubes,  and  bottles  of  chemicals,  especially 
of  specimens  of  pure  metals,  lined  its  walls. 

The  observatory  became  a  meeting  place  where  ter- 
restrial chemistry  was  brought  into  direct  touch  with  celes- 
tial chemistry.  The  characteristic  light-rays  from  earthly 
hydrogen  shone  side  by  side  with  the  corresponding  radia- 
tions from  starry  hydrogen,  or  else  fell  upon  the  dark  lines 
due  to  the  absorption  of  the  hydrogen  in  Sirius  or  in  Vega. 
Iron  from  our  mines  was  line-matched,  light  for  dark,  with 
stellar  iron  from  opposite  parts  of  the  celestial  sphere. 
Sodium,  which  upon  the  earth  is  always  present  with  us, 
was  found  to  be  widely  diffused  through  the  celestial 
spaces. 

This  time  was,  indeed,  one  of  strained  expectation  and 
of  scientific  exaltation  for  the  astronomer,  almost  without 
parallel;  for  nearly  every  observation  revealed  a  new  fact, 
and  almost  every  night's  work  was  red-lettered  by  some  dis- 
covery. And  yet,  notwithstanding,  we  had  to  record  that 
"  the  inquiry  in  which  we  have  been  engaged  has  been  more 
than  usually  toilsome;  indeed,  it  has  demanded  a  sacrifice 
of  time  very  great  when  compared  with  the  amount  of  in- 
formation which  we  have  been  able  to  obtain." 

Soon  after  the  close  of  1862  we  sent  a  preliminary  note 
to  the  Royal  Society,  "  On  the  Lines  of  some  of  the  Fixed 
Stars,"  in  which  we  gave  diagrams  of  the  spectra  of  Sirius, 
Betelgeux,  and  Aldebaran,  with  the  statement  that  we  had 
observed  the  spectra  of  some  forty  stars,  and  also  the  spectra 
of  the  planets  Jupiter  and  Mars.  It  was  a  little  remarkable 
that  on  the  same  day  on  which  our  paper  was  to  be  read, 
but  some  little  time  after  it  had  been  sent  in,  news  arrived 
there  from  America  that  similar  observations  on  some  of 
the  stars  had  been  made  by  Mr.  Rutherfurd.  A  very  little 


THE   NEW  ASTRONOMY  451 

later  similar  work  on  the  spectra  of  the  stars  was  undertaken 
in  Rome  by  Secchi,  and  in  Germany  by  Vogel. 

In  February,  1863,  the  strictly  astronomical  character 
of  the  observatory  was  further  encroached  upon  by  the 
erection,  in  one  corner,  of  a  small  photographic  tent  fur- 
nished with  baths  and  other  appliances  for  the  wet  collodion 
process.  We  obtained  photographs,  indeed,  of  the  spectra 
of  Sirius  and  Capella;  but  from  want  of  steadiness  and  more 
perfect  adjustment  of  the  instruments,  the  spectra,  though 
defined  at  the  edges,  did  not  show  the  dark  lines  as  we  ex- 
pected. The  dry  collodion  plates  then  available  were  not 
rapid  enough;  and  the  wet  process  was  so  inconvenient 
for  long  exposures,  from  irregular  drying,  and  draining 
back  from  the  positions  in  which  the  plates  had  often  to 
be  put,  that  we  did  not  persevere  in  our  attempts  to  photo- 
graph the  stellar  spectra.  I  resumed  them  with  success  in 
1875,  as  we  shall  see  further  on. 

At  that  time  no  convenient  maps  of  the  spectra  of  the 
chemical  elements,  which  were  then  but  imperfectly  known, 
were  available  for  comparison  with  the  spectra  of  the  stars. 
Kirchhoff's  maps  were  confined  to  a  few  elements,  and 
were  laid  down  on  an  arbitrary  scale,  relatively  to  the  solar 
spectrum.  It  was  not  always  easy,  since  our  work  had  to 
be  done  at  night  when  the  solar  spectrum  could  not  be 
seen,  to  recognise  with  certainty  even  the  lines  included  in 
Kirchhoff's  maps.  To  meet  this  want,  I  devoted  a  great 
part  of  1863  to  mapping,  with  a  train  of  six  prisms,  the  spec- 
tra of  twenty-six  of  the  elements;  using  as  a  standard  scale 
the  spark-spectrum  of  common  air,  which  would  be  always 
at  hand.  The  lines  of  air  were  first  carefully  referred  to 
those  of  purified  oxygen  and  nitrogen.  The  spectra  were 
obtained  by  the  discharge  of  a  large  induction  coil  furnished 
with  a  condenser  of  several  Leyden  jars.  I  was  much 
assisted  by  specimens  of  pure  metals  furnished  to  me  by 
Dr.  W.  A.  Miller  and  Dr.  Matthiessen.  My  paper  on  this 
subject,  and  its  accompanying  maps,  appeared  in  the  vol- 
ume of  the  Transactions  of  the  Royal  Society  for  1864. 

During  the  same  time,  whenever  the  nights  were  fine, 


452  HUGGINS 

our  work  on  the  spectra  of  the  stars  went  on,  and  the  re- 
sults were  communicated  to  the  Royal  Society  in  April, 
1864,  after  which  Dr.  Miller  had  not  sufficient  leisure  to 
continue  working  with  me.  The  general  accuracy  of  our 
work,  so  far  as  it  was  possible  with  the  instruments  at  our 
disposal,  is  shown  by  the  good  agreement  of  the  spectra 
of  Aldebaran  and  Betelgeux  with  the  observations  of  the 
same  stars  made  later  in  Germany  by  Vogel. 

It  is  obviously  unsafe  to  claim  for  spectrum  compari- 
sons a  greater  degree  of  accuracy  than  is  justified  by  the 
resolving  power  employed.  When  the  apparent  coinci- 
dences of  the  lines  of  the  same  substance  are  numerous, 
as  in  the  case  of  iron;  or  the  lines  are  characteristically 
grouped,  as  are  those  of  hydrogen,  of  sodium,  and  of  mag- 
nesium, there  is  no  room  for  doubt  that  the  same  substances 
are  really  in  the  stars.  Coincidence  with  a  single  line  may 
be  little  better  than  trusting  to  a  bruised  reed;  for  the  stellar 
line  may,  under  greater  resolving  power,  break  up  into  two 
or  more  lines,  and  then  the  coincidence  may  disappear.  As 
we  shall  see  presently,  the  apparent  position  of  the  star  line 
may  not  be  its  true  one,  in  consequence  of  the  earth's  or 
the  star's  motion  in  the  line  of  sight.  Our  work,  however, 
was  amply  sufficient  to  give  a  certain  reply  to  the  wonder 
that  had  so  long  asked  in  vain  of  what  the  stars  were  made. 
The  chemistry  of  the  solar  system  was  shown  to  prevail, 
essentially  at  least,  wherever  a  star  twinkles.  The  stars  were 
undoubtedly  suns  after  the  order  of  our  sun,  though  not  at 
all  at  the  same  evolutional  stage,  older  or  younger  it  may 
be,  in  the  life  history  of  bodies  of  which  the  vitality  is  heat. 
Further,  elements  which  play  a  chief  role  in  terrestrial 
physics,  as  iron,  hydrogen,  sodium,  magnesium,  calcium, 
were  found  to  be  the  first  and  the  most  easily  recognised  of 
the  earthly  substances  in  the  stars. 

Soon  after  the  completion  of  the  joint  work  of  Dr. 
Miller  and  myself,  and  then  working  alone,  I  was  fortunate 
in  the  early  autumn  of  the  same  year,  1864,  to  begin  some 
observations  in  a  region  hitherto  unexplored;  and  which, 
to  this  day,  remain  associated  in  my  memory  with  the  pro- 


THE   NEW   ASTRONOMY 


453 


found  awe  which  I  felt  on  looking  for  the  first  time  at  that 
which  no  eye  of  man  had  seen,  and  which  even  the  sci- 
entific imagination  could  not  foreshow. 

The  attempt  seemed  almost  hopeless.  For  not  only  are 
the  nebulae  very  faintly  luminous — as  Marius  put  it,  "  like  a 
rush-light  shining  through  a  horn  " — but  their  feeble  shin- 
ing can  not  be  increased  in  brightness,  as  can  be  that  of 
the  stars,  neither  to  the  eye  nor  in  the  spectroscope,  by 
any  optic  tube,  however  great. 

Shortly  after  making  the  observations  of  which  I  am 
about  to  speak,  I  dined  at  Greenwich,  Otto  Struve  being 
also  a  guest,  when,  on  telling  of  my  recent  work  on  the 
nebulae,  Sir  George  Airy  said,  "  It  seems  to  me  a  case  of 
'  Eyes  and  No  Eyes.' '  Such  work,  indeed,  it  was,  as  we 
shall  see,  on  certain  of  the  nebulae. 

The  nature  of  these  mysterious  bodies  was  still  an  un- 
read riddle.  Toward  the  end  of  the  last  century  the  elder 
Herschel,  from  his  observations  at  Slough,  came  very  near 
suggesting  what  is  doubtless  the  true  nature,  and  place  in 
the  Cosmos,  of  the  nebulae.  I  will  let  him  speak  in  his  own 
words: 

"  A  shining  fluid  of  a  nature  unknown  to  us. 

"  What  a  field  of  novelty  is  here  opened  to  our  con- 
ceptions! .  .  .  We  may  now  explain  that  very  extensive 
nebulosity,  expanded  over  more  than  sixty  degrees  of  the 
heavens,  about  the  constellation  of  Orion;  a  luminous  mat- 
ter accounting  much  better  for  it  than  clustering  stars  at 
a  distance.  .  .  . 

"  If  this  matter  is  self-luminous,  it  seems  more  fit  to 
produce  a  star  by  its  condensation  than  to  depend  on  the 
star  for  its  existence." 

This  view  of  the  nebulae  as  parts  of  a  fiery  mist  out  of 
which  the  heavens  had  been  slowly  fashioned,  began,  a 
little  before  the  middle  of  the  present  century,  at  least  in 
many  minds,  to  give  way  before  the  revelations  of  the  giant 
telescopes  which  had  come  into  use,  and  especially  of  the 
telescope,  six  feet  in  diameter,  constructed  by  the  late  Earl 
of  Rosse  at  a  cost  of  not  less  than  £12,000. 


454 


HUGGINS 


Nebula  after  nebula  yielded,  being  resolved  apparently 
into  innumerable  stars,  as  the  optical  power  was  increased; 
and  so  the  opinion  began  to  gain  ground  that  all  nebulae 
may  be  capable  of  resolution  into  stars.  According  to  this 
view,  nebulae  would  have  to  be  regarded,  not  as  early  stages 
of  an  evolutional  progress,  but  rather  as  stellar  galaxies 
already  formed,  external  to  our  system — cosmical  "  sand- 
heaps  "  too  remote  to  be  separated  into  their  component 
stars.  Lord  Rosse  himself  was  careful  to  point  out  that 
it  would  be  unsafe  from  his  observations  to  conclude  that 
all  nebulosity  is  but  the  glare  of  stars  too  remote  to  be 
resolved  by  our  instruments.  In  1858  Herbert  Spencer 
showed  clearly  that,  notwithstanding  the  Parsonstown  reve- 
lations, the  evidence  from  the  observation  of  nebulae  up  to 
that  time  was  really  in  favour  of  their  being  early  stages 
of  an  evolutional  progression. 

On  the  evening  of  the  2Qth  of  August,  1864,  I  directed 
the  telescope  for  the  first  time  to  a  planetary  nebula  in 
Draco.  The  reader  may  now  be  able  to  picture  to  him- 
self to  some  extent  the  feeling  of  excited  suspense, 
mingled  with  a  degree  of  awe,  with  which,  after  a  few 
moments  of  hesitation,  I  put  my  eye  to  the  spectro- 
scope. Was  I  not  about  to  look  into  a  secret  place  of 
creation? 

I  looked  into  the  spectroscope.  No  spectrum  such  as 
I  expected!  A  single  bright  line  only!  At  first  I  suspected 
some  displacement  of  the  prism,  and  that  I  was  looking  at 
a  reflection  of  the  illuminated  slit  from  one  of  its  faces. 
This  thought  was  scarcely  more  than  momentary;  then  the 
true  interpretation  flashed  upon  me.  The  light  of  the 
nebula  was  monochromatic,  and  so,  unlike  any  other  light 
I  had  as  yet  subjected  to  prismatic  examination,  could  not 
be  extended  out  to  form  a  complete  spectrum.  After  pass- 
ing through  the  two  prisms  it  remained  concentrated  into 
a  single  bright  line,  having  a  width  corresponding  to  the 
width  of  the  slit,  and  occupying  in  the  instrument  a  posi- 
tion at  that  part  of  the  spectrum  to  which  its  light 
belongs  in  refrangibility.  A  little  closer  looking  showed 


THE   NEW   ASTRONOMY  455 

two  other  bright  lines  on  the  side  toward  the  blue,  all 
the  three  lines  being  separated  by  intervals  relatively 
dark. 

The  riddle  of  the  nebulae  was  solved.  The  answer, 
which  had  come  to  us  in  the  light  itself,  read:  Not  an  ag- 
gregation of  stars,  but  a  luminous  gas.  Stars  after  the 
order  of  our  own  sun,  and  of  the  brighter  stars,  would  give 
a  different  spectrum;  the  light  of  this  nebula  had  clearly 
been  emitted  by  a  luminous  gas.  With  an  excess  of  cau- 
tion, at  the  moment  I  did  not  venture  to  go  further  than 
to  point  out  that  we  had  here  to  do  with  bodies  of  an  order 
quite  different  from  that  of  the  stars.  Further  observations 
soon  convinced  me  that,  though  the  short  span  of  human 
life  is  far  too  minute  relatively  to  cosmical  events  for  us 
to  expect  to  see  in  succession  any  distinct  steps  in  so  august 
a  process,  the  probability  is  indeed  overwhelming  in  favour 
of  an  evolution  in  the  past,  and  still  going  on,  of  the 
heavenly  hosts.  A  time  surely  existed  when  the  matter  now 
condensed  into  the  sun  and  planets  filled  the  whole  space 
occupied  by  the  solar  system,  in  the  condition  of  gas,  which 
then  appeared  as  a  glowing  nebula,  after  the  order,  it  may 
be,  of  some  now  existing  in  the  heavens.  There  remained 
no  room  for  doubt  that  the  nebulae,  which  our  telescopes 
reveal  to  us,  are  the  early  stages  of  long  processions  of 
cosmical  events,  which  correspond  broadly  to  those  re- 
quired by  the  nebular  hypothesis  in  one  or  other  of  its 
forms. 

Not,  indeed,  that  the  philosophical  astronomer  would 
venture  to  dogmatize  in  matters  of  detail,  or  profess  to  be 
able  to  tell  you  pat  off  by  heart  exactly  how  everything 
has  taken  place  in  the  universe,  with  the  flippant  tongue 
of  a  Lady  Constance  after  reading  "  The  Revelations  of 
Chaos": 

"  It  shows  you  exactly  how  a  star  is  formed;  nothing 
could  be  so  pretty.  A  cluster  of  vapour — the  cream  of 
'the  Milky  Way;  a  sort  of  celestial  cheese  churned  into 
light." 

It  is  necessary  to  bear  distinctly  in  mind  that  the  old 


456  HUGGINS 

view  which  made  the  matter  of  the  nebulae  to  consist  of  an 
original  fiery  mist — in  the  words  of  the  poet: 

"...  a  tumultuous  cloud 
Instinct  with  fire  and  nitre  " — 

could  no  longer  hold  its  place  after  Helmholtz  had  shown, 
in  1854,  that  such  an  originally  fiery  condition  of  the  nebu- 
lous stuff  was  quite  unnecessary,  since  in  the  mutual  gravi- 
tation of  widely  separated  matter  we  have  a  store  of  poten- 
tial energy  sufficient  to  generate  the  high  temperature  of 
the  sun  and  stars. 

The  solution  of  the  primary  riddle  of  the  nebulae  left 
pending  some  secondary  questions.  What  chemical  sub- 
stances are  represented  by  the  newly  found  bright  lines? 
Is  solar  matter  common  to  the  nebulae  as  well  as  to  the 
stars?  What  are  the  physical  conditions  of  the  nebulous 
matter? 

Further  observations  showed  two  lines  of  hydrogen; 
and  recent  observations  have  shown  associated  with  it  the 
new  element  recently  discovered  by  Professor  Ramsay,  oc- 
cluded in  certain  minerals,  and  of  which  a  brilliant  yellow 
line  in  the  sun  had  long  been  looked  upon  as  the  badge  of 
an  element  as  yet  unknown.  The  principal  line  of  these 
nebulae  suggests  probably  another  substance  which  has  not 
yet  been  unearthed  from  its  hiding  place  in  terrestrial  rocks 
by  the  cunning  of  the  chemist. 

Are  the  nebulae  very  hot,  or  comparatively  cool?  The 
spectroscope  indicates  a  high  temperature — that  is  to  say, 
that  the  individual  molecules  or  atoms,  which  by  their  en- 
counters are  luminous,  have  motions  corresponding  to  a 
very  high  temperature,  and  in  this  sense  are  very  hot.  On 
account  of  the  great  extent  of  the  nebulae,  however,  a  com- 
paratively small  number  of  luminous  molecules  might  be 
sufficient  to  make  them  as  bright  as  they  appear  to  us;  tak- 
ing this  view,  their  mean  temperature,  if  they  can  be  said 
to  have  one,  might  be  low,  and  so  correspond  with  what 
we  might  expect  to  find  in  gaseous  masses  at  an  early 
stage  of  condensation. 


THE   NEW  ASTRONOMY 


457 


In  the  nebulae  I  had  as  yet  examined  the  condensa- 
tion of  nearly  all  the  light  into  a  few  bright  lines  made  the 
observations  of  their  spectra  less  difficult  than  I  feared 
would  be  the  case.  It  became,  indeed,  a  case  of  "  Eyes  and 
No  Eyes  "  when  a  few  days  later  I  turned  the  telescope  to 
the  great  nebula  in  Andromeda.  Its  light  was  distributed 
throughout  the  spectrum,  and  consequently  extremely 
faint.  The  brighter  middle  part  only  could  be  seen,  though 
I  have  since  proved,  as  I  at  first  suggested  might  be  the 
case,  that  the  blue  and  the  red  ends  are  really  not  absent, 
but  are  not  seen  on  account  of  their  feebler  effect  upon  the 
eye.  Though  continuous,  the  spectrum  did  not  look  uni- 
form in  brightness,  but  its  extreme  feebleness  made  it  un- 
certain whether  the  irregularities  were  due  to  certain  parts 
being  enhanced  by  bright  lines,  or  the  other  parts  enfeebled 
by  dark  lines. 

Out  of  sixty  of  the  brighter  nebulas  and  clusters,  I  found 
about  one  third,  including  the  planetary  nebulae  and  that 
of  Orion,  to  give  the  bright-line  spectrum.  It  would  be 
altogether  out  of  place  here  to  follow  the  results  of  my 
further  observations  along  the  same  lines  of  research,  which 
occupied  the  two  years  immediately  succeeding. 

I  pass  at  once  to  a  primary  spectroscopic  observation  of 
one  of  those  rare  and  strange  sights  of  the  heavens,  of 
which  only  about  nineteen  have  been  recorded  in  as  many 
centuries: 

"...  those  far  stars  that  come  in  sight 
Once  in  a  century." 

On  the  1 8th  of  May,  1866,  at  5  P.  M.,  a  letter  came  with 
the  address  "  Tuam,  from  an  unknown  correspondent,  one 
John  Birmingham."  Mr.  Birmingham  afterward  became 
well  known  by  his  observations  of  variable  stars,  and  espe- 
cially by  his  valuable  catalogue  of  Red  Stars  in  1877.  The 
letter  ran: 

"  I  beg  to  direct  your  attention  to  a  new  star  which  I 
observed  last  Saturday  night,  and  which  must  be  a  most 


458  HUGGINS 

interesting  object  for  spectrum  analysis.  It  is  situated  in 
Cor.  Bor.,  and  is  very  brilliant,  of  about  the  second  magni- 
tude. I  sent  an  account  of  it  to  the  '  Times '  yesterday, 
but  as  that  journal  is  not  likely  to  publish  communications 
from  this  part  of  the  world,  I  scarcely  think  that  it  will  find 
a  place  for  mine." 

Fortunately  the  evening  was  fine,  and  as  soon  as  it  was 
dusk  I  looked,  with  not  a  little  scepticism,  I  freely  confess, 
at  the  place  of  the  sky  named  in  the  letter.  To  my  great 
joy,  there  shone  a  bright  new  star,  giving  a  new  aspect  to 
the  Northern  Crown;  of  the  order  doubtless  of  the  splen- 
did temporary  star  of  1572,  which  Tycho  supposed  to  be 
generated  from  the  ethereal  substance  of  the  Milky  Way, 
and  afterward  dissipated  by  the  sun,  or  dissolved  from  some 
internal  cause. 

I  sent  a  messenger  for  my  friend  Dr.  Miller,  and  an 
hour  later  we  directed  the  telescope,  with  spectroscope 
attached,  to  the  blazing  star.  Later  in  the  evening  a  letter 
arrived  from  Mr.  Baxendale,  who  had  independently  dis- 
covered the  star  on  the  I5th. 

By  this  evening,  the  i8th,  the  star  had  already  fallen  in 
brightness  below  the  third  magnitude.  The  view  in  the 
spectroscope  was  strange,  and  up  to  that  time  unprece- 
dented. Upon  a  spectrum  of  the  solar  order,  with  its  num- 
berless dark  lines,  shone  out  brilliantly  a  few  very  bright 
lines.  There  was  little  doubt  that  at  least  two  of  these  lines 
belonged  to  hydrogen.  The  great  brilliancy  of  these  lines 
as  compared  with  the  parts  of  the  continuous  spectrum 
upon  which  they  fell  suggested  a  temperature  for  the  gas 
emitting  them  higher  than  that  of  the  star's  photosphere. 

Few  of  days,  as  indeed  had  been  its  forbears  appearing 
at  long  intervals,  the  new  star  waned  with  a  rapidity  little 
less  remarkable  than  was  the  suddenness  of  its  outburst, 
without  visible  descent,  all  armed  in  a  full  panoply  of  light 
from  the  moment  of  its  birth.  A  few  hours  only  before 
Birmingham  saw  it  blazing  with  second-magnitude  splen- 
dour, Schmidt,  observing  at  Athens,  could  testify  that  no 
outburst  had  taken  place.  Rapid  was  the  decline  of  its 


THE   NEW  ASTRONOMY  459 

light,  falling  in  twelve  days  from  the  second  down  to  the 
eighth  magnitude. 

It  was  obvious  to  us  that  no  very  considerable  mass  of 
matter  could  cool  down  from  the  high  temperature  indi- 
cated by  the  bright  lines  in  so  short  a  time.  At  the  same 
time  it  was  not  less  clear  that  the  extent  of  the  mass  of  the 
fervid  gas  must  be  on  a  very  grand  scale  indeed,  for  a  star 
at  its  undoubted  distance  from  us,  to  take  on  so  great  a 
splendour.  These  considerations  led  us  to  suggest  some 
sudden  and  vast  convulsion,  which  had  taken  place  in  a  star 
so  far  cooled  down  as  to  give  but  little  light,  or  even  to  be 
partially  crusted  over,  by  volcanic  forces,  or  by  the  disturb- 
ing approach  or  partial  collision  of  another  dark  star.  The 
essential  character  of  the  explanation  lay  in  the  suggestion 
of  a  possible  chemical  combination  of  some  of  the  escaping 
highly  heated  gases  from  within,  when  cooled  by  the  sud- 
den expansion,  which  might  give  rise  to  an  outburst  of 
flame  at  once  very  brilliant  and  of  very  short  duration. 

The  more  precise  statement  of  what  occurred  during 
our  observations,  as  made  afterward  from  the  pulpit  of  one 
of  our  cathedrals — "  that  from  afar  astronomers  had  seen 
a  world  on  fire  go  out  in  smoke  and  ashes  " — must  be  put 
down  to  an  excess  of  the  theological  imagination. 

From  the  beginning  of  our  work  upon  the  spectra  of 
the  stars  I  saw  in  vision  the  application  of  the  new  knowl- 
edge to  the  creation  of  a  great  method  of  astronomical  ob- 
servation which  could  not  fail  in  future  to  have  a  powerful 
influence  on  the  progress  of  astronomy;  indeed,  in  some 
respects  greater  than  the  more  direct  one  of  the  investiga- 
tion of  the  chemical  nature  and  the  relative  physical  con- 
ditions of  the  stars. 

It  was  the  opprobrium  of  the  older  astronomy — though, 
indeed,  one  which  involved  no  disgrace,  for  a  1'impossible 
nul  n'est  tenu — that  only  that  part  of  the  motions  of  the 
stars  which  is  across  the  line  of  sight  could  be  seen  and  di- 
rectly measured.  The  direct  observation  of  the  other  com- 
ponent in  the  line  of  sight,  since  it  caused  no  change  of 
place  and,  from  the  great  distance  of  the  stars,  no  appre- 


460  HUGGINS 

ciable  change  of  size  or  of  brightness  within  an  observer's 
lifetime,  seemed  to  lie  hopelessly  quite  outside  the  limits  of 
man's  powers.  Still,  it  was  only  too  clear  that,  so  long  as 
we  were  unable  to  ascertain  directly  those  components  of 
the  star's  motions  which  lie  in  the  line  of  sight,  the  speed 
and  direction  of  the  solar  motion  in  space,  and  many  of  the 
great  problems  of  the  constitution  of  the  heavens,  must 
remain  more  or  less  imperfectly  known. 

Now,  as  the  colour  of  a  given  kind  of  light,  and  the  ex- 
act position  it  would  take  up  in  a  spectrum,  depends  directly 
upon  the  length  of  the  waves,  or,  to  put  it  differently,  upon 
the  number  of  waves  which  would  pass  into  the  eye  in  a 
second  of  time,  it  seemed  more  than  probable  that  motion 
between  the  source  of  the  light  and  the  observer  must 
change  the  apparent  length  of  the  waves  to  him,  and  the 
number  reaching  his  eye  in  a  second.  To  a  swimmer  strik- 
ing out  from  the  shore  each  wave  is  shorter,  and  the  num- 
ber he  goes  through  in  a  given  time  is  greater  than  would 
be  the  case  if  he  had  stood  still  in  the  water.  Such  a  change 
of  wave-length  would  transform  any  given  kind  of  light,  so 
that  it  would  take  a  new  place  in  the  spectrum,  and  from 
the  amount  of  this  change  to  a  higher  or  to  a  lower  place, 
we  could  determine  the  velocity  per  second  of  the  relative 
motion  between  the  star  and  the  earth. 

The  notion  that  the  propagation  of  light  is  not  instan- 
taneous, though  rapid  far  beyond  the  appreciation  of  our 
senses,  is  due,  not,  as  is  sometimes  stated,  to  Francis,  but 
to  Roger  Bacon.  "  Relinquitur  ergo,"  he  says,  in  his 
"  Opus  Majus,"  "  quod  lux  multiplicatur  in  tempore  .  .  . 
sed  tamen  non  in  tempore  sensibili  et  perceptibili  a  visu, 
sed  insensibili.  .  .  ."  The  discovery  of  its  actual  velocity 
was  made  by  Roemer,  in  1675,  from  observations  of  the 
satellites  of  Jupiter.  Now,  though  the  effect  of  motion  in 
the  line  of  sight  upon  the  apparent  velocity  of  light  under- 
lies Roemer's  determinations,  the  idea  of  a  change  of  colour 
in  light  from  motion  between  the  source  of  light  and  the 
observer  was  announced  for  the  first  time  by  Doppler  in 
1841.  Later,  various  experiments  were  made  in  connection 


THE   NEW  ASTRONOMY  461 

•with  this  view  by  Ballot,  Sestini,  Klinkerfues,  Clerk  Max- 
well, and  Fizeau.  But  no  attempts  had  been  made,  nor 
were  indeed  possible,  to  discover  by  this  principle  the  mo- 
tions of  the  heavenly  bodies  in  the  line  of  sight.  For,  to 
learn  whether  any  change  in  the  light  had  taken  place  from 
motion  in  the  line  of  sight,  it  was  clearly  necessary  to  know 
the  original  wave-length  of  the  light  before  it  left  the  star. 

As  soon  as  our  observations  had  shown  that  certain 
earthly  substances  were  present  in  the  stars,  the  original 
wave-lengths  of  their  lines  became  known,  and  any  small 
want  of  coincidence  of  the  stellar  lines  with  the  same  lines 
produced  upon  the  earth  might  safely  be  interpreted  as 
revealing  the  velocity  of  approach  or  of  recession  between 
the  star  and  the  earth. 

These  considerations  were  present  to  my  mind  from  the 
first,  and  helped  me  to  bear  up  under  many  toilsome  disap- 
pointments: "  Studio  fallente  laborem."  It  was  not  until 
1866  that  I  found  time  to  construct  a  spectroscope  of 
greater  power  for  this  research.  It  would  be  scarcely  pos- 
sible, even  with  greater  space,  to  convey  to  the  reader  any 
true  conception  of  the  difficulties  which  presented  them- 
selves in  this  work,  from  various  instrumental  causes,  and 
of  the  extreme  care  and  caution  which  were  needful  to  dis- 
tinguish spurious  instrumental  shifts  of  a  line  from  a  true 
shift  due  to  the  star's  motion. 

At  last,  in  1868,  I  felt  able  to  announce  in  a  paper 
printed  in  the  "  Transactions  "  of  the  Royal  Society  for  that 
year  the  foundation  of  this  new  method  of  research,  which, 
transcending  the  wildest  dreams  of  an  earlier  time,  enables 
the  astronomer  to  measure  off  directly  in  terrestrial  units 
the  invisible  motions  in  the  line  of  sight  of  the  heavenly 
bodies. 

To  pure  astronomers  the  method  came  before  its  time, 
since  they  were  then  unfamiliar  with  Spectrum  Analysis, 
which  lay  completely  outside  the  routine  work  of  an  ob- 
servatory. It  would  be  easy  to  mention  the  names  of  men 
well  known,  to  whom  I  was  "  as  a  very  lovely  song  of  one 
that  hath  a  pleasant  voice."  They  heard  my  words,  but  for 


462  HUGGINS 

a  time  were  very  slow  to  avail  themselves  of  this  new  power 
of  research.  My  observations  were,  however,  shortly  after- 
ward confirmed  by  Vogel  in  Germany,  and  by  others  the 
principle  was  soon  applied  to  solar  phenomena.  By  mak- 
ing use  of  improved  methods  of  photography,  Vogel  has 
recently  determined  the  motions  of  approach  and  of  reces- 
sion of  some  fifty  stars,  with  an  accuracy  of  about  an  Eng- 
lish mile  a  second.  In  the  hands  of  Young,  Duner,  Keeler, 
and  others,  the  method  has  been  successfully  applied  to  a 
determination  of  the  rotation  of  the  sun,  of  Saturn  and  his 
rings,  and  of  Jupiter. 

It  has  become  fruitful  in  another  direction,  for  it  puts 
into  our  hands  the  power  of  separating  double  stars  which 
are  beyond  the  resolving  power  of  any  telescope  that  can 
ever  be  constructed.  Pickering  and  Vogel  have  independ- 
ently discovered  by  this  method  an  entirely  new  class  of 
double  stars. 

Double  stars  too  close  to  be  separately  visible  unite  in 
giving  a  compound  spectrum.  Now,  if  the  stars  are  in 
motion  about  a  common  centre  of  gravity,  the  lines  of  one 
star  will  shift  periodically  relatively  to  similar  lines  of  the 
other  star,  in  the  spectrum  common  to  both;  and  such  lines 
will  consequently,  at  those  times,  appear  double.  Even  if 
one  of  the  stars  is  too  dark  to  give  a  spectrum  which  can 
be  seen  upon  that  of  the  other  star,  as  is  actually  the  case 
with  Algol  and  Spica,  the  whirling  of  the  stars  about  each 
other  may  be  discovered  from  the  periodical  shifting  of  the 
lines  of  the  brighter  star  relatively  to  terrestrial  lines  of  the 
same  substance.  It  is  clear  that  as  the  stars  revolve  about 
their  common  centre  of  gravity,  the  bright  star  would  be 
sometimes  advancing,  and  at  others  receding,  relatively  to 
an  observer  on  the  earth,  except  it  should  so  happen  that 
the  stars'  orbit  were  perpendicular  to  the  line  of  sight. 

It  would  be  scarcely  possible,  without  the  appearance 
of  great  exaggeration,  to  attempt  to  sketch  out  even  in 
broad  outline  the  many  glorious  achievements  which 
doubtless  lie  before  this  method  of  research  in  the  imme- 
diate future. 


THE   NEW  ASTRONOMY  463 

Comets  in  the  olden  time  were  looked  upon  as  the  por- 
tents of  all  kinds  of  woe: 

"  There  with  long  bloody  haire,  a  blazing  star 
Threatens  the  World  with  Famin,  Plague,  and  War." 

Though  they  were  no  longer,  at  the  time  of  which  I  am 
speaking,  a  terror  to  mankind,  they  were  a  great  mystery. 
Perhaps  of  no  other  phenomenon  of  Nature  had  so  many 
guesses  at  truth  been  made  on  different,  and  even  on  oppos- 
ing principles  of  explanation.  It  was  about  this  time  that 
a  beam  of  light  was  thrown  in,  for  the  first  time,  upon  the 
night  of  mystery  in  which  they  moved  and  had  their  being, 
by  the  researches  of  Newton  of  Yale  College,  by  Adams, 
and  by  Schiaparelli.  The  unexpected  fact  came  out  of  the 
close  relationship  of  the  orbits  of  certain  comets  with  those 
of  periodic  meteor-swarms.  Only  a  year  before  the  obser- 
vations of  which  I  am  about  to  speak  were  made,  Odling 
had  lighted  up  the  theatre  of  the  Royal  Institution  with 
gas  brought  by  a  meteorite  from  celestial  space.  Two 
years  earlier,  Donati  showed  the  light  of  a  small  comet  to 
be  in  part  self-emitted,  and  so  not  wholly  reflected  sun- 
shine. 

I  had  myself,  in  the  case  of  three  faint  comets,  in  1866, 
in  1867,  and  January,  1868,  discovered  that  part  of  their 
light  was  peculiar  to  them,  and  that  the  light  of  the  last  one 
consisted  mainly  of  three  bright  flutings.  Intense,  there- 
fore, was  the  great  expectancy  with  which  I  directed  the 
telescope  with  its  attached  spectroscope  to  the  much 
brighter  comet  which  appeared  in  June,  1868. 

The  comet's  light  was  resolved  into  a  spectrum  of  three 
bright  bands  or  flutings,  each  alike  falling  off  in  brightness 
on  the  more  refrangible  side.  On  the  evening  of  the  22d  I 
measured  the  positions  in  the  spectrum  of  the  brighter  be- 
ginnings of  the  flutings  on  the  red  side.  I  was  not  a  little 
surprised  the  next  morning  to  find  that  the  three  cometary 
flutings  agreed  in  position  with  three  similar  flutings  in  the 
brightest  part  of  the  spectrum  of  carbon.  Some  time  before, 
I  had  mapped  clown  the  spectrum  of  carbon,  from  different 


464  HUGGINS 

sources,  chiefly  from  different  hydrocarbons.  In  some  of 
these  spectra,  the  separate  lines  of  which  the  flutings  are 
built  up  are  individually  more  distinct  than  in  others.  The 
comet  bands,  as  I  had  seen  them  on  the  previous  evening, 
appeared  to  be  identical  in  character  in  this  respect,  as  well 
as  in  position  in  the  spectrum,  with  the  flutings  as  they 
appeared  when  I  took  the  spark  in  a  current  of  olefiant 
gas.  I  immediately  filled  a  small  holder  with  this  gas,  ar- 
ranged an  apparatus  in  such  a  manner  that  the  gas  could  be 
attached  to  the  end  of  the  telescope,  and  its  spectrum,  when 
a  spark  was  taken  in  it,  seen  side  by  side  with  that  of  the 
comet. 

Fortunately  the  evening  was  fine;  and  on  account  of 
the  exceptional  interest  of  confronting  for  the  first  time  the 
spectrum  of  an  earthly  gas  with  that  of  a  comet's  light,  I 
invited  Dr.  Miller  to  come  and  make  the  crucial  observa- 
tion with  me.  The  expectation  which  I  had  formed  from 
my  measures  was  fully  confirmed.  The  comet's  spectrum 
when  seen  together  with  that  from  the  gas  agreed  in  all 
respects  precisely  with  it.  The  comet,  though  "  subtle  as 
Sphinx,"  had  at  last  yielded  up  its  secret.  The  principal 
part  of  its  light  was  emitted  by  luminous  vapour  of  carbon. 

This  result  was  in  harmony  with  the  nature  of  the  gas 
found  occluded  in  meteorites.  Odling  had  found  carbonic 
oxide  as  well  as  hydrogen  in  his  meteorite.  Wright,  ex- 
perimenting with  another  type  of  meteorite,  found  that 
carbon  dioxide  was  chiefly  given  off.  Many  meteorites 
contain  a  large  percentage  of  hydrocarbons;  from  one  of 
such  sky-stones  a  little  later  I  observed  a  spectrum  similar 
to  that  of  the  comet.  The  three  bands  may  be  seen  in 
the  base  of  a  candle  flame. 

Since  these  early  observations  the  spectra  of  many 
comets  have  been  examined  by  many  observers.  The  close 
general  agreement  as  to  the  three  bright  flutings  which 
form  the  main  feature  of  the  cometary  spectrum  confirms 
beyond  doubt  the  view  that  the  greater  part  of  the  light  of 
comets  is  due  to  the  fluted  spectrum  of  carbon.  Some 
additional  knowledge  of  the  spectra  of  comets,  obtained 


THE   NEW   ASTRONOMY  465 

by  means  of  photography,  will  have  its  proper  place 
later  on. 

About  this  time  I  devoted  some  attention  to  spectro- 
scopic  observations  of  the  sun,  and  especially  to  the  modi- 
fications of  the  spectrum  which  take  place  under  the  in- 
fluence of  the  solar  spots. 

The  aerial  ocean  around  and  above  us,  in  which  finely 
divided  matter  is  always  more  or  less  floating,  becomes  itself 
illuminated,  and  a  source  of  light,  when  the  sun  shines  upon 
it,  and  so  conceals,  like  a  luminous  veil,  any  object  less 
brilliant  than  itself  in  the  heavens  beyond.  From  this  cause 
the  stars  are  invisible  at  midday.  This  curtain  of  light 
above  us  at  all  ordinary  times  shuts  out  from  our  view  the 
magnificent  spectacle  of  red  flames  flashing  upon  a  coronal 
glory  of  bright  beams  and  streamers,  which  suddenly  bursts 
upon  the  sight,  for  a  few  minutes  only,  when  at  rare  inter- 
vals the  light  curtain  is  lifted  by  the  screening  of  the  sun's 
light  by  the  moon,  at  a  total  eclipse. 

As  yet  the  spectrum  of  the  red  flames  had  not  been 
seen.  If,  as  seemed  probable,  it  should  be  found  to  be  that 
of  a  gas,  consisting  of  bright  lines  only,  it  was  conceivable 
that  the  spectroscope  might  enable  us  so  to  weaken  by  dis- 
persion the  air-glare,  relatively  to  the  bright  lines  which 
would  remain  undispersed,  that  the  bright  lines  of  the 
flames  might  become  visible  through  the  atmospheric 
glare. 

The  historic  sequence  of  events  is  as  follows.  In  No- 
vember, 1866,  Mr.  Lockyer  asked  the  question:  "  May  not 
the  spectroscope  afford  us  evidence  of  the  existence  of  the 
red  flames,  which  total  eclipses  have  revealed  to  us  in  the 
sun's  atmosphere;  though  they  escape  all  other  methods  of 
observation  at  other  times?  " 

In  the  "  Report  of  the  Council  of  the  Royal  Astronomi- 
cal Society,"  read  in  February,  1868,  occurs  the  following 
statement,  furnished  by  me,  in  which  the  explanation  is 
fully  given  of  the  principle  on  which  I  had  been  working 
to  obtain  the  spectrum  of  the  red  flames  without  an  eclipse: 

"  During  the  last  two  years  Mr.   Huggins  has  made 


i*=> 
30 


466  HUGGINS 

numerous  observations  for  the  purpose  of  obtaining  a  view, 
if  possible,  of  the  red  prominences  seen  during  an  eclipse. 
The  invisibility  of  these  objects  at  ordinary  times  is  sup- 
posed to  arise  from  the  illumination  of  our  atmosphere.  If 
these  bodies  are  gaseous,  their  spectra  would  consist  of 
bright  lines.  With  a  powerful  spectroscope  the  light  re- 
flected from  our  atmosphere  near  the  sun's  limb  edge  would 
be  greatly  reduced  in  intensity  by  the  dispersion  of  the 
prisms,  while  the  bright  lines  of  the  prominences,  if  such  be 
present,  would  remain  but  little  diminished  in  brilliancy. 
This  principle  has  been  carried  out  by  various  forms  of 
prismatic  apparatus,  and  also  by  other  contrivances,  but 
hitherto  without  success." 

At  the  total  eclipse  of  the  sun,  August  18,  1868,  sev- 
eral observers  saw  the  light  of  the  red  flames  to  be  resolved 
in  their  spectroscopes  into  bright  lines,  among  which  lines 
of  hydrogen  were  recognised.  The  distinguished  astron- 
omer, Janssen,  one  of  the  observers  in  India,  saw  some  of 
the  bright  lines  again  the  next  day,  by  means  of  the  prin- 
ciple described  above,  when  there  was  no  eclipse. 

On  October  2Qth  Mr.  Lockyer  sent  a  note  to  the  Royal 
Society  to  say  that  on  that  day  he  had  succeeded  in  ob- 
serving three  bright  lines,  of  a  fine  prominence. 

About  the  time  that  the  news  of  the  discovery  of  the 
bright  lines  at  the  eclipse  reached  this  country,  in  Sep- 
tember, I  was  altogether  incapacitated  for  work  for  some 
little  time  through  the  death  of  my  beloved  mother.  We 
had  been  all  in  all  to  each  other  for  many  years.  The  first 
day  I  was  sufficiently  recovered  to  resume  work,  Decem- 
ber iQth,  on  looking  at  the  sun's  limb  with  the  same  spec- 
troscope I  had  often  used  before,  now  that  I  knew  exactly 
at  what  part  of  the  spectrum  to  search  for  the  lines,  I  saw 
them  at  the  first  moment  of  putting  my  eye  to  the  instru- 
ment. 

As  yet,  by  all  observers  the  lines  only  of  the  promi- 
nences had  been  seen,  and  therefore  to  learn  their  forms 
it  was  necessary  to  combine  in  one  design  the  lengths  of  the 
lines  as  they  varied,  when  the  slit  was  made  to  pass  over  a 


THE   NEW   ASTRONOMY 


467 


prominence.  In  February  of  the  following  year  it  occurred 
to  me  that  by  widening  the  opening  of  the  slit  the  form 
of  a  prominence,  and  not  its  lines  only,  might  be  directly 
observed.  This  method  of  using  a  wide  slit  has  been  since 
universally  employed. 

It  does  not  fall  within  the  scope  of  this  article  to  de- 
scribe an  ingenious  photographic  method  by  which  Hale 
has  been  able  to  take  daily  records  of  the  constantly  vary- 
ing phenomena  of  the  red  flames  and  the  bright  faculae, 
upon  and  around  the  solar  disk. 

The  purpose  of  this  article  is  to  sketch,  in  very  broad 
outline  only,  the  principal  events,  in  the  order  of  their  suc- 
cession in  time,  quorum  pars  magna  fui,  which  contributed 
in  an  important  degree  to  the  rise  of  the  new  astronomy. 
As  a  science  advances  it  follows  naturally  that  its  further 
progress  will  consist  more  and  more  in  matters  of  detail, 
and  in  points  which  are  of  technical  rather  than  of  general 
interest. 

It  would,  therefore,  be  altogether  out  of  place  here  to 
carry  on  in  detail  the  narrative  of  the  work  of  my  observa- 
tory, when,  as  was  inevitable,  it  began  to  take  on  the 
character  of  a  development  only,  along  lines  of  which  I 
have  already  spoken — namely,  the  observation  of  more 
stars,  and  of  other  nebulae,  and  other  comets.  I  pass  on  at 
once,  therefore,  to  the  year  1876,  in  which  by  the  aid  of 
the  new  dry  plates,  with  gelatine  films,  introduced  by  Mr. 
Kennett,  I  was  able  to  take  up  again,  and  this  time  with 
success,  the  photography  of  the  spectra  of  the  stars,  of 
my  early  attempts  at  which  I  have  already  spoken. 

I  was  now  better  prepared  for  work.  My  observatory 
had  been  enlarged  from  a  dome  of  twelve  feet  in  diameter 
to  a  drum  having  a  diameter  of  eighteen  feet.  This  altera- 
tion had  been  made  for  the  reception  of  a  larger  telescope 
made  by  Sir  Howard  Grubb,  at  the  expense  of  a  legacy  to 
the  Royal  Society,  and  which  was  placed  in  my  hands  on 
loan  by  that  society.  This  instrument  was  furnished  with 
two  telescopes:  an  achromatic  of  fifteen  inches  aperture, 
and  a  Cassegrain  of  eighteen  inches  aperture,  with  mirrors 


468  HUGGINS 

of  speculum  metal.  At  this  time,  one  only  of  these  tele- 
scopes could  be  in  use  at  a  time.  Later  on,  in  1882,  by  a 
device  which  occurred  to  me  of  giving  each  telescope  an 
independent  polar  axis,  the  one  working  within  the  other, 
both  telescopes  could  remain  together  on  the  equatorial 
mounting,  and  be  equally  ready  for  use. 

By  this  time  I  had  the  great  happiness  of  having  se- 
cured an  able  and  enthusiastic  assistant  by  my  marriage 
in  1875. 

The  great  and  notable  advances  in  astronomical  meth- 
ods and  discoveries  by  means  of  photography  since  1875 
are  due  almost  entirely  to  the  great  advantages  which  the 
gelatine  dry  plate  possesses  for  use  in  the  observatory,  over 
the  process  of  Daguerre,  and  even  over  that  of  wet  collo- 
dion. The  silver-bromide  gelatine  plate,  which  I  was  the 
first,  I  believe,  to  use  for  photographing  the  spectra  of 
stars,  except  for  its  grained  texture,  meets  the  need  of  the 
astronomer  at  all  points.  This  plate  possesses  extreme  sen- 
sitiveness; it  is  always  ready  for  use;  it  can  be  placed  in 
any  position;  it  can  be  exposed  for  hours;  lastly,  imme- 
diate development  is  not  necessary,  and  for  this  reason,  as 
I  soon  found  to  be  necessary  in  this  climate,  it  can  be  ex- 
posed again  to  the  same  object  on  succeeding  nights;  and 
so  make  up  by  successive  instalments,  as  the  weather  may 
permit,  the  total  long  exposure  which  may  be  needful. 

The  power  of  the  eye  falls  off  as  the  spectrum  extends 
beyond  the  blue,  and  soon  fails  altogether.  There  is  there- 
fore no  drawback  to  the  use  of  glass  for  the  prisms  and 
lenses  of  a  visual  spectroscope.  But  while  the  sensitiveness 
of  a  photographic  plate  is  not  similarly  limited,  glass,  like  the 
eye,  is  imperfectly  transparent,  and  soon  becomes  opaque, 
to  the  parts  of  the  spectrum  at  a  short  distance  beyond  the 
limit  of  the  visible  spectrum.  To  obtain,  therefore,  upon 
the  plate  a  spectrum  complete  at  the  blue  end  of  stellar 
light,  it  was  necessary  to  avoid  glass,  and  to  employ  instead 
Iceland  spar  and  rock  crystal,  which  are  transparent  up  to 
the  limit  of  the  ultra-violet  light  which  can  reach  us  through 
our  atmosphere.  Such  a  spectroscope  was  constructed  and 


THE   NEW   ASTRONOMY  469 

fixed  with  its  slit  at  the  focus  of  the  great  speculum  of  the 
Cassegrain  telescope. 

How  was  the  image  of  a  star  to  be  easily  brought,  and 
then  kept,  for  an  hour  or  even  for  many  hours,  precisely  at 
one  place  on  a  slit  so  narrow  as  about  the  y^  of  an  inch? 
For  this  purpose  the  very  convenient  device  was  adopted 
of  making  the  slit-plates  of  highly  polished  metal,  so  as  to 
form  a  divided  mirror,  in  which  the  reflected  image  of  a 
star  could  be  observed  from  the  eye  end  of  the  telescope 
by  means  of  a  small  telescope  fixed  within  the  central  hole 
of  the  great  mirror.  A  photograph  of  the  spectrum  of 
a  Lyrae,  taken  with  this  instrument,  was  shown  at  the  Royal 
Society  in  1876. 

In  the  spectra  of  such  stars  as  Sirius  and  Vega  there 
came  out  in  the  ultra-violet  region,  which  up  to  that  time 
had  remained  unexplored,  the  completion  of  a  grand  rhyth- 
mical group  of  strong  dark  lines,  of  which  the  well-known 
hydrogen  lines  in  the  visible  region  form  the  lower  mem- 
bers. Terrestrial  chemistry  became  enriched  with  a  more 
complete  knowledge  of  the  spectrum  of  hydrogen  from  the 
stars.  Shortly  afterward  Cornu  succeeded  in  photograph- 
ing a  similar  spectrum  in  his  laboratory  from  earthly  hy- 
drogen. 

I  presented  in  1879  a  paper,  with  maps,  to  the  Royal 
Society,  on  the  photographic  spectra  of  the  stars,  which  was 
printed  in  their  "  Transactions  "  for  1880.  In  this  paper, 
besides  descriptions  of  the  photographs,  and  tables  of  the 
measures  of  the  positions  of  the  lines,  I  made  a  first  at- 
tempt to  arrange  the  stars  in  a  possible  evolutional  series 
from  the  relative  behaviour  of  the  hydrogen  and  the  me- 
tallic lines.  In  this  series  Sirius  and  Vega  are  placed  at 
the  hotter  and  earlier  end;  Capella  and  the  sun  at  about 
the  same  evolutional  stage,  somewhere  in  the  middle  of 
the  series;  while  at  the  most  advanced  and  oldest  stage  of 
the  stars  which  I  had  then  photographed  came  Betelgeux, 
in  the  spectrum  of  which  the  ultra-violet  region,  though 
not  wanting,  is  very  greatly  enfeebled. 

Shortly  afterward  I  directed  the  photographic  arrange- 


470  HUGGINS 

ment  of  combined  spectroscope  and  telescope  to  the  nebula 
in  Orion,  and  obtained  for  the  first  time  information  of  the 
nature  of  its  spectrum  beyond  the  visible  region.  One  line 
a  little  distance  on  in  the  ultra-violet  region  came  out  very 
strongly  on  the  plate.  If  this  kind  of  light  came  within  the 
range  of  our  vision  it  would  no  doubt  give  the  dominant 
colour  to  the  nebula,  in  place  of  its  present  blue-greenish 
hue.  Other  lines  of  the  hydrogen  series,  as  might  be  ex- 
pected, were  seen  in  the  photograph,  together  with  a  num- 
ber of  other  bright  lines. 

In  1 88 1,  for  the  first  time  since  the  spectroscope  and 
also  suitable  photographic  plates  had  been  in  the  hands  of 
astronomers,  the  coming  of  a  bright  comet  made  it  possible 
to  extend  the  examination  of  its  light  into  the  invisible  re- 
gion of  the  spectrum  at  the  blue  end.  On  the  22d  of  June, 
by  leaving  very  early  a  banquet  at  the  Mansion  House,  I 
was  able,  after  my  return  home,  to  obtain,  with  an  exposure 
of  one  hour,  a  good  photograph  of  the  head  of  the  comet. 
It  was  under  a  great  tension  of  expectancy  that  the  plate 
was  developed,  so  that  I  might  be  able  to  look  for  the  first 
time  into  a  virgin  region  of  Nature,  as  yet  unexplored  by 
the  eye  of  man. 

The  plate  contained  an  extension  and  confirmation  of 
my  earlier  observations  by  eye.  There  were  the  combined 
spectra  of  two  kinds  of  light — a  faint  continuous  spectrum, 
crossed  by  Fraunhofer  lines  which  showed  it  to  be  reflected 
solar  light.  Upon  this  was  seen  a  second  spectrum  of  the 
original  light  emitted  by  the  comet  itself.  This  spectrum 
consisted  mainly  of  two  groups  of  bright  lines,  character- 
istic of  the  spectra  of  certain  compounds  of  carbon.  It  will 
be  remembered  that  my  earlier  observations  revealed  the 
three  principal  flutings  of  carbon  as  the  main  feature  of  a 
comet's  spectrum  in  the  visible  region.  The  photograph 
brought  a  new  fact  to  light.  Liveing  and  Dewar  had  shown 
that  one  of  these  bands  consisted  of  lines  belonging  to  a 
nitrogen  compound  of  carbon.  We  gained  the  new  knowl- 
edge that  nitrogen,  as  well  as  carbon  and  hydrogen,  exists 
in  comets.  Now,  nitrogen  is  present  in  the  gas  found 


THE   NEW   ASTRONOMY 


471 


occluded  in  some  meteorites.  At  a  later  date,  Dr.  Flight 
showed  that  nitrogen  formed  as  much  as  seventeen  per  cent 
of  the  occluded  gas  from  the  meteorite  of  Cranbourne, 
Australia. 

I  have  now  advanced  to  the  extreme  limit  of  time  within 
which  the  rise  of  the  New  Astronomy  can  be  regarded  as 
taking  place.  At  this  time,  in  respect  of  the  broad  lines 
of  its  methods,  and  the  wide  scope  of  the  directions  in 
which  it  was  already  applied,  it  had  become  well  estab- 
lished. Already  it  possessed  a  literature  of  its  own,  and 
many  observatories  were  becoming,  in  part  at  least,  devoted 
to  its  methods. 

In  my  own  observatory  work  has  gone  on  whenever  our 
unfavourable  climate  has  permitted  observations  to  be 
made.  At  the  present  moment  more  than  one  research  is 
in  progress.  It  would  be  altogether  beyond  the  intention 
and  limited  scope  of  the  present  article  to  follow  this  later 
work. 

We  found  the  New  Astronomy  newly  born  in  a  labora- 
tory at  Heidelberg;  to  astronomers  she  was 

"...  a  stranger, 
Born  out  of  their  dominions." 

We  take  leave  of  her  in  the  full  beauty  of  a  vigorous  youth, 
receiving  homage  in  nearly  all  the  observatories  of  the 
world,  some  of  which,  indeed,  are  devoted  wholly  to  her 
cult.  So  powerful  is  the  magic  of  her  charms  that  gifts  have 
poured  in  from  all  sides  to  do  her  honour.  It  has  been  by 
such  free  gifts  that  Pickering,  at  Cambridge,  United  States, 
and  in  the  southern  hemisphere,  has  been  able  to  give  her 
so  devoted  a  service.  In  this  country,  where  from  almost 
the  hour  of  her  birth  she  won  hearts,  enthusiastic  wor- 
shippers have  not  been  wanting.  By  the  liberality  of  the 
late  Mr.  Newall,  and  the  disinterested  devotion  of  his  son, 
a  well-equipped  observatory  is  now  wholly  given  up  to  her 
worship  at  Cambridge.  This  Jubilee  year  is  red-lettered  at 
Greenwich  by  the  inauguration  of  a  magnificent  double 
telescope,  laid  at  her  feet  by  Sir  Henry  Thompson.  Next 


472  HUGGINS 

year  the  Royal  Observatory  at  the  Cape  will  be  able  to  add 
to  its  devotion  to  the  old  astronomy  a  homage  not  less  sin- 
cere and  enthusiastic  to  the  New  Astronomy,  by  means  of 
the  splendid  instruments  which  Mr.  McClean,  who  person- 
ally serves  under  her  colours,  has  presented  to  that  ob- 
servatory. In  Germany,  the  first  National  Observatory 
dedicated  to  the  New  Astronomy  in  1874,  under  the  direc- 
tion of  the  distinguished  astrophysicist,  Professor  Vogel,  is 
about  to  be  furnished  by  the  Government  with  new  and 
larger  instruments  in  her  honour. 

In  America  many  have  done  liberally,  but  Mr.  Yerkes 
has  excelled  them  all.  This  summer  will  be  celebrated  the 
opening  of  a  palatial  institution  on  the  shore  of  Lake  Ge- 
neva founded  by  Mr.  Yerkes,  and  dedicated  to  our  fair  lady, 
the  New  Astronomy.  This  observatory,  in  respect  of  the 
great  size  of  its  telescope,  of  forty  inches  in  aperture,  the 
largest  yet  constructed,  its  armoury  of  instruments  for  spec- 
troscopic  attack  upon  the  heavens,  and  the  completeness 
of  its  laboratories  and  its  workshops,  will  represent  the 
most  advanced  state  of  instrument-making;  and  at  the  same 
time  render  possible,  under  the  most  favourable  conditions, 
the  latest  and  the  most  perfect  methods  of  research  of  the 
New  Astronomy.  Above  all,  the  needful  men  will  not  be 
wanting.  A  knightly  band,  who  have  shown  their  knight- 
hood by  prowess  in  discovery,  led  by  Professor  Hale  in 
chivalrous  quest  of  Truth,  will  surely  make  this  palace  of 
the  New  Astronomy  worthy  to  be  regarded  as  the  Urani- 
borg  of  the  end  of  the  nineteenth  century,  as  the  Danish 
Observatory,  under  Tycho  and  his  astronomers,  repre- 
sented the  highest  development  of  astronomy  at  the  close 
of  the  sixteenth. 


AN  ASTRONOMER'S  WORK  IN  A 
MODERN  OBSERVATORY 


BY 

DAVID   GILL 


AN   ASTRONOMER'S  WORK   IN  A 
MODERN   OBSERVATORY1 

THE  work  of  astronomical  observatories  has  been  di- 
vided into  two  classes — viz.,  astrometry  and  astro- 
physics. The  first  of  these  relates  to  astronomy  of 
precision — that  is,  to  the  determination  of  the  position  of 
celestial  objects;  the  second  relates  to  the  study  of  their 
physical  features  and  chemical  construction. 

Some  years  ago  the  aims  and  objects  of  these  two 
classes  of  observatories  might  have  been  considered  per- 
fectly distinct,  and  in  fact  were  so  considered.  But  I  hope 
to  show  that  in  more  recent  years  their  objects  and  their 
processes  have  become  so  interlaced  that  they  can  not  with 
advantage  be  divided,  and  a  fully  equipped  modern  ob- 
servatory must  be  understood  to  include  the  work  both  of 
astrometry  and  astrophysics. 

In  any  such  observatory  the  principal  and  the  funda- 
mental instrument  is  the  transit  circle.  It  is  upon  the  posi- 
tion in  the  heavens  of  celestial  objects,  as  determined  with 
this  instrument  or  with  kindred  instruments,  that  the  whole 
fair  superstructure  of  exact  astronomy  rests — that  is  to  say, 
all  that  we  find  of  information  and  prediction  in  our  nau- 
tical almanacs,  all  that  we  know  of  the  past  and  can  pre- 
dict for  the  future  motions  of  the  celestial  bodies. 

Here  is  a  very  small  and  imperfect  model,  but  it  will 
serve  to  render  intelligible  the  photographs  of  the  actual 
instrument  which  will  be  subsequently  projected  on  the 
screen.  (Here  the  lecturer  described  the  adjustments  and 
mode  of  using  a  transit  circle.) 

1  From  the  "  Proceedings  of  the  Royal  Institution  of  Great  Britain," 
i8oo-'92. 

475 


476  GILL 

We  are  now  in  a  position  to  understand  photographs 
of  the  instrument  itself.  But  first  of  all  as  to  the  house  in 
which  it  dwells.  Here,  now  on  the  screen,  is  the  outside 
of  the  main  building  of  the  Royal  Observatory,  Cape  of 
Good  Hope.  I  select  it  simply  because,  being  the  observa- 
tory which  it  is  my  privilege  to  direct,  it  is  the  one  of 
which  I  can  most  easily  procure  a  series  of  photographs. 
It  was  built  during  the  years  1824-^28,  and  like  all  the 
observatories  built  about  that  time,  and  like  too  many  built 
since,  it  is  a  very  fair  type  of  most  of  the  things  which  an 
observatory  should  not  be.  It  is,  as  you  see,  an  admirably 
solid  and  substantial  structure,  innocent  of  any  architec- 
tural charm,  and  so  far  as  it  affords  an  excellent  dwelling- 
place,  good  library  accommodations,  and  good  rooms 
for  computers,  no  fault  can  be  found  with  it.  But  these 
very  qualities  render  it  undesirable  as  an  observatory.  An 
essential  matter  for  a  perfect  observatory  should  be  the 
possibility  to  equalize  the  internal  and  external  tempera- 
ture. The  site  of  an  instrument  should  also  be  free  from 
the  immediate  surroundings  of  chimneys  or  other  origin 
of  ascending  currents  of  heated  air.  Both  these  conditions 
are  incompatible  with  thick  walls  of  masonry  and  the  chim- 
neys of  attached  dwelling-houses,  and  therefore,  as  far  as 
possible,  I  have  removed  the  instruments  to  small  detached 
houses  of  their  own.  But  the  transit  circle  still  remains 
in  the  main  building,  for,  as  will  be  evident  to  you,  it  is 
no  easy  matter  to  transport  such  an  instrument. 

The  first  two  photographs  show  the  instrument  in  one 
case  pointed  nearly  horizontally  to  the  north,  the  other 
pointed  nearly  vertical.  Neither  can  show  all  parts  of  the 
instrument,  but  you  can  see  the  massive  stone  piers,  weigh- 
ing many  tons  each,  which,  resting  on  the  solid  blocks  ten 
feet  below,  support  the  pivots.  Here  are  the  counter- 
weights, which  remove  a  great  part  of  the  weight  of  the 
instrument  from  the  pivots,  leaving  only  a  residual  pres- 
sure sufficient  to  enable  the  pivots  to  preserve  the  motion 
of  the  instrument  in  its  proper  plane.  Here  are  the  micro- 
scopes by  which  the  circle  is  read;  here  the  openings 


WORK   IN   A   MODERN   OBSERVATORY 


477 


through  which  the  instrument  views  the  meridian  sky. 
The  observer's  chair  is  shown  in  this  diagram.  His  work 
appears  to  be  very  simple,  and  so  it  is,  but  it  requires  spe- 
cial natural  gifts — patience  and  devotion,  and  a  high  sense 
of  the  importance  of  his  work — to  make  a  first-class  me- 
ridian observer.  Nothing  apparently  more  monotonous 
can  be  well  imagined  if  a  man  is  not  "  to  the  manner  born." 

Having  directed  this  instrument  by  means  of  the  setting 
circle  to  the  required  altitude,  he  clamps  it  there  and  waits 
for  the  star  which  he  is  about  to  observe  to  enter  the  field. 

As  the  star  enters  the  field  it  passes  wire  after  wire,  and 
as  it  passes  each  wire  he  presses  the  key  of  his  chrono- 
graph and  records  the  instant  automatically.  As  the  star 
passes  the  middle  wire  he  bisects  it  with  the  horizontal 
web,  and  again  similarly  records  on  his  chronograph  the 
transit  of  the  star  over  the  remaining  webs.  Then  he  reads 
off  the  microscopes  by  which  the  circle  is  read,  and  also 
the  barometer  and  thermometer,  in  order  afterward  to  be 
able  to  calculate  accurately  the  effect  of  atmospheric  re- 
fraction on  the  observed  altitude  of  the  star;  and  then  his 
observation  is  finished.  Thus  the  work  of  the  meridian 
observer  goes  on,  star  after  star,  hour  after  hour,  and  night 
after  night;  and,  as  you  see,  it  differs  very  widely  from  the 
popular  notion  of  an  astronomer's  occupation.  It  presents 
no  dreamy  contemplation,  no  watching  for  new  stars,  no 
unexpected  or  startling  phenomena.  On  the  contrary, 
there  is  beside  him  the  carefully  prepared  observing-list  for 
the  night,  the  previously  calculated  circle  setting  for  each 
star,  allowing  just  sufficient  time  for  the  new  setting  for 
the  next  star  after  the  readings  of  the  circle  for  the  pre- 
vious observation. 

After  four  or  five  hours  of  this  work  the  observers  have 
had  enough  of  it;  they  have,  perhaps,  observed  fifty  or 
sixty  stars,  they  determine  certain  instrumental  errors,  and 
betake  themselves  to  bed,  tired  but  (if  they  are  of  the  right 
stuff)  happy  and  contented  men.  At  the  Cape  we  employ 
two  observers — the  one  to  read  the  circle  and  the  other  to 
record  the  transit.  Four  observers  are  employed,  and  they 


478  GILL 

are  thus  on  duty  each  alternate  night.  Such  is  the  work 
that  an  outsider  would  see  were  he  to  enter  a  working 
meridian  observatory  at  night;  but  he  would  find  out  if 
he  came  next  morning  that  the  work  was  by  no  means 
over.  By  far  the  largest  part  has  yet  to  follow.  An  ob- 
servation that  requires  only  two  or  three  minutes  to  make 
at  night  requires  at  least  half  an  hour  for  its  reduction  by 
day.  Each  observation  is  affected  by  a  number  of  errors, 
and  these  have  to  be  determined  and  allowed  for.  Al- 
though solidly  founded  on  massive  piers  resting  on  solid 
rock,  the  constancy  of  the  instrument's  position  can  not 
be  relied  upon.  It  goes  through  small  periodic  changes 
in  level,  in  collimation,  and  in  azimuth,  which  have  to  be 
determined  by  proper  means,  and  the  corresponding  cor- 
rections have  to  be  computed  and  applied;  and  also  there 
are  other  corrections  for  refraction,  etc.,  which  involve 
computation  and  have  to  be  applied.  But  these  matters 
would  fall  more  properly  under  the  head  of  a  special  lec- 
ture upon  the  transit  instrument.  I  mention  them  now 
merely  to  explain  why  so  great  a  part  of  an  astronomer's 
work  comes  in  the  daytime,  and  to  dispel  the  notion  that 
his  work  belongs  only  to  the  night. 

One  might  very  well  occupy  a  special  lecture  in  an  ac- 
count of  the  peculiarities  of  what  is  called  personal  equa- 
tion— that  is  to  say,  the  different  time  which  elapses  for 
different  observers  between  the  time  when  the  observer 
believes  the  star  to  be  upon  the  wire  and  the  time  when 
the  finger  responds  to  the  message  which  the  eye  has  con- 
veyed to  the  brain.  Some  observers  always  press  the  key 
too  soon,  some  always  too  late.  Some  years  ago  I  dis- 
covered, from  the  observations  to  which  I  will  subsequently 
refer,  that  all  observers  press  the  chronograph  key  either 
too  soon  for  bright  stars  or  too  late  for  faint  ones. 

Other  errors  may,  and  I  am  sure  do,  arise  both  at 
Greenwich  and  the  Cape  from  the  impossibility  of  securing 
uniformity  of  outside  and  inside  temperature  in  a  building 
of  strong  masonry.  The  ideal  observatory  should  be  as  solid 
as  possible  as  to  its  foundations,  but  as  light  as  possible  as  to 


WORK   IN   A   MODERN   OBSERVATORY  479 

its  roof  and  walls — say,  a  light  framework  of  iron  covered 
with  canvas.  But  it  would  be  undesirable  to  cover  a  valu- 
able and  permanent  instrument  in  this  way. 

But  here  is  a  form  of  observatory  which  realizes  all 
that  is  required,  and  which  is  eminently  suited  for  per- 
manent use.  The  walls  are  of  sheet  iron,  which  readily 
acquire  the  temperature  of  the  outer  air.  The  iron  walls 
are  protected  from  direct  sunshine  by  wooden  louvres,  and 
small  doors  in  the  iron  walls  admit  a  free  circulation  of 
air.  The  revolving  roof  is  a  light  framework  of  iron  cov- 
ered with  well-painted  papier-mache. 

The  photograph  now  on  the  screen  shows  the  interior 
of  the  observatory,  and  this  brings  me  to  the  description 
of  observations  of  an  entirely  different  class.  In  this  ob- 
servatory the  roof  turns  round  on  wheels,  so  that  any  part 
of  the  sky  can  be  viewed  from  the  telescope.  This  is  so 
because  the  instrument  in  this  observatory  is  intended  for 
purposes  which  are  entirely  different  from  those  of  a  tran- 
sit circle.  The  transit  circle,  as  we  have  seen,  is  used  to 
determine  the  absolute  positions  of  the  heavenly  bodies; 
the  heliometer  to  determine  with  greater  precision  than  is 
possible  by  the  absolute  method  the  relative  positions  of 
celestial  objects. 

To  explain  my  meaning  as  to  absolute  and  relative  posi- 
tions: It  would,  for  example,  be  a  matter  of  very  little  im- 
portance if  the  absolute  latitude  of  a  point  on  the  Royal 
Exchange  or  the  Bank  of  England  were  one  tenth  of  a 
second  of  arc  (or  ten  feet)  wrong  in  the  maps  of  the 
Ordnance  Survey  of  England — that  would  constitute  a 
small  absolute  error  common  to  all  buildings  on  the  same 
map  of  a  part  of  the  city,  and  common  to  all  the  adjoining 
maps  also.  Such  an  error,  regarded  as  an  absolute  error, 
would  evidently  be  of  no  importance  if  every  point  on  the 
map  had  the  same  absolute  error.  There  is  no  one  who 
can  say  at  the  present  moment  whether  the  absolute  lati- 
tude of  the  Royal  Exchange — nay,  even  of  the  Royal  Ob- 
servatory, Greenwich — is  known  to  ten  feet.  But  it  would 
be  a  very  serious  thing  indeed  if  the  relative  positions  on 


48o  GILL 

the  same  map  were  ten  feet  wrong  here  and  there.  For 
example,  if  of  two  points  marking  a  frontage  boundary  on 
Cornhill  one  were  correct,  the  other  ten  feet  in  error — 
what  a  nice  fuss  there  would  be!  what  food  for  lawyers! 
what  a  bad  time  for  the  Ordnance-Survey  Office!  Well, 
it  is  just  the  same  in  astronomy. 

We  do  not  know,  we  probably  never  shall  know  with 
certainty,  the  absolute  places  of  even  the  principal  stars 
to  one  tenth  of  a  second  of  arc.  But  one  tenth  of  a  second 
of  arc  in  the  measure  of  some  relative  position  would  be 
fatal.  For  example,  in  the  measurement  of  the  sun's  paral- 
lax an  error  of  one  tenth  of  a  second  of  arc  means  an  error 
of  one  million  miles,  in  round  numbers,  in  the  sun's  dis- 
tance; and  it  is  only  when  we  can  be  quite  certain  of  our 
measures  of  much  smaller  quantities  than  one  tenth  of  a 
second  of  arc  that  we  are  in  a  position  to  begin  seriously 
the  determination  of  such  a  problem  as  that  of  the  distance 
of  the  fixed  stars.  For  these  problems  we  must  use  differ- 
ential measures — that  is,  measures  for  the  relative  positions 
of  two  objects.  The  most  perfect  instrument  for  such  pur- 
poses is  the  heliometer. 

Lord  McLaren  has  kindly  sent  from  Edinburgh,  for 
the  purposes  of  this  lecture,  the  parts  of  his  heliometer 
which  are  necessary  to  illustrate  the  principles  of  the  in- 
strument. 

This  instrument  is  the  same  which  I  used  on  Lord 
Crawford's  expedition  to  Mauritius  in  1874.  It  was  also 
kindly  loaned  me  by  Lord  Crawford  for  an  expedition  to 
the  Island  of  Ascension  to  observe  the  opposition  of  Mars 
in  1877.  In  1879,  when  I  went  to  the  Cape,  I  acquired 
the  instrument  from  Lord  Crawford,  and  carried  out  cer- 
tain researches  with  it  on  the  distances  of  the  fixed  stars. 

In  1887,  when  the  Admiralty  provided  the  new  heli- 
ometer for  the  Cape  Observatory,  this  instrument  again 
changed  hands.  It  became  the  property  of  Lord  McLaren. 
I  felt  rather  disloyal  in  parting  with  so  old  a  friend.  We 
had  spent  so  many  happy  hours  together,  we  had  shared 
a  good  many  anxieties  together,  and  we  knew  each  other's 


WORK   IN   A   MODERN   OBSERVATORY  481 

weaknesses  so  well!  But  my  old  friend  has  fallen  into  good 
hands,  and  has  found  another  sphere  of  work. 

There  is  now  on  the  screen  a  picture  of  the  new  heli- 
ometer  of  the  Cape  Observatory,  which  was  mounted  in 
1887,  and  has  been  in  constant  use  ever  since.  It  is  an 
instrument  of  the  most  refined  modern  construction,  and 
is  probably  the  finest  apparatus  for  refined  measurements 
of  celestial  angles  in  the  world. 

(Here  were  explained  the  various  parts  of  the  instru- 
ment in  relation  to  the  model,  and  the  actual  processes  of 
observation  were  illustrated  by  the  images  of  artificial  stars 
projected  on  a  screen.) 

Here,  again,  there  is  little  that  conforms  to  the  popular 
idea  of  an  astronomer's  work;  there  is  no  searching  for 
objects,  no  contemplative  watching,  nothing  sensational  of 
any  kind.  On  the  contrary,  every  detail  of  his  work  has 
been  previously  arranged  and  calculated  beforehand,  and 
the  prospect  that  lies  before  him  in  his  night's  work  is 
simply  more  or  less  of  a  struggle  with  the  difficulties  which 
are  created  by  the  agitation  of  the  star  images,  caused  by 
irregularities  in  the  atmospheric  refraction.  It  is  not  upon 
one  night  in  a  hundred  that  the  images  of  stars  are  per- 
fectly tranquil.  You  have  the  same  effect  in  an  exagger- 
ated way  when  looking  across  a  bog  on  a  hot  day.  Thus, 
generally,  as  the  images  are  approached,  they  appear  to 
cross  and  recross  each  other,  and  the  observer  must  either 
seize  a  moment  of  comparative  tranquility  to  make  his 
definitive  bisection,  or  he  may  arrive  at  it  by  gradual  ap- 
proximations till  he  finds  that  the  vibrating  images  of  the 
two  stars  seem  to  pass  each  other  as  often  to  one  side  as 
to  the  other.  So  soon  as  such  a  bisection  has  been  made 
the  time  is  recorded  on  the  chronograph,  then  the  scales 
are  pointed  on  and  printed  off,  and  so  the  work  goes  on, 
varied  only  by  reversals  of  the  segments  and  of  the  posi- 
tion circle.  Generally  I  now  arrange  for  thirty-two  such 
bisections,  and  these  occupy  about  an  hour  and  a  half. 
By  that  time  one  has  had  about  enough  of  it;  the  nerves 
are  somewhat  tired,  so  are  the  muscles  of  the  back  of  the 
31 


482  GILL 

neck,  and,  if  the  observer  is  wise  and  wishes  to  do  his 
best  work,  he  goes  to  bed  early  and  gets  up  again  at  two 
or  three  o'clock  in  the  morning  and  goes  through  a  similar 
piece  of  work.  In  fact,  this  must  be  his  regular  routine 
night  after  night,  whenever  the  weather  is  clear,  if  he  is 
engaged,  as  I  have  been,  on  a  large  programme  of  work 
on  the  parallaxes  of  the  fixed  stars,  or  on  observations  to 
determine  the  distance  of  the  sun  by  observations  of  minor 
planets. 

I  will  not  speak  now  of  these  researches,  because  they 
are  still  in  progress  of  execution  or  of  reduction.  I  would 
rather,  in  the  first  place,  endeavour  to  complete  the  pic- 
ture of  a  night's  work  in  a  modern  observatory. 

We  pass  on  to  celestial  photography,  where  astrometry 
and  astrophysics  join  hands.  The  observer's  work  duijjng 
the  exposure  is  simply  to  direct  the  telescope  to  the  re- 
quired part  of  the  sky,  and  then  the  clockwork  nearly  does 
the  rest — but  not  quite  so.  The  observer  holds  in  his 
hand  a  little  electrical  switch  with  two  keys;  by  pressing 
one  key  he  can  accelerate  the  velocity  of  the  driving  screw 
by  about  one  per  cent,  and  by  pressing  the  other  he  can 
retard  it  one  per  cent.  In  this  way  he  keeps  one  of  the 
stars  in  the  field  always  perfectly  bisected  by  the  cross-wires 
of  his  guiding  telescope,  and  thus  corrects  the  small  errors 
produced  partly  by  changes  of  refraction,  partly  by  small 
unavoidable  errors  in  cutting  the  teeth  of  the  arc  into  which 
the  screw  of  the  driving  shaft  of  the  clockwork  gears. 

The  work  is  monotonous  rather  than  fatiguing,  and  the 
companionship  of  a  pipe  or  cigar  is  very  helpful  during 
long  exposures.  A  man  can  go  on  for  a  watch  of  four  or 
five  hours  very  well,  taking  plate  after  plate,  exposing  each, 
it  may  be,  forty  minutes  or  an  hour.  If  the  night  is  fine 
a  second  observer  follows  the  first,  and  so  the  work  goes 
on  the  greater  part  of  the  night.  Next  day  he  develops 
his  plate. 

Working  just  in  this  way,  but  with  the  more  humble 
apparatus  which  you  see  imperfectly  in  the  picture  now 
on  the  screen,  we  have,  with  a  rapid  rectilinear  lens  by  Ball- 


WORK   IN   A  MODERN  OBSERVATORY  483 

meyer  of  six  inches  aperture,  photographed  at  the  Cape 
during  the  past  six  years  the  whole  of  the  southern  hemi- 
sphere from  20°  of  south  declination  to  the  south  pole. 

The  plates  are  being  measured  by  Professor  Kapteyn, 
of  Groningen,  and  I  expect  that  in  the  course  of  a  year  the 
whole  work  containing  all  the  stars  to  9^  magnitude  (be- 
tween 200,000  and  300,000  stars)  in  that  region  will  be 
ready  for  publication.  This  work  is  essential  as  a  pre- 
liminary step  for  the  execution  in  the  southern  hemisphere 
of  the  great  work  inaugurated  by  the  Astrophotographic 
Congress  at  Paris  in  1887,  the  last  details  of  which  were 
settled  at  our  meeting  at  Paris  in  April  last.  What  we 
shall  do  with  the  new  apparatus  perhaps  I  may  have  the 
honour  to  describe  to  you  some  years  hence,  after  the  work 
bas  been  done. 

We  now  come  to  an  important  class  of  astronomical 
work  more  purely  astrophysical,  for  the  illustration  of 
which  I  can  no  longer  appeal  to  the  Cape,  because  I  re- 
gret to  say  that  we  are  not  yet  provided  with  the  means 
for  its  prosecution.  I  refer  to  the  use  of  the  spectroscope 
in  astronomy,  and  especially  to  the  latest  developments  of 
its  use  for  the  accurate  measurement  of  the  velocity  of  the 
motions  of  stars  in  the  line  of  sight.2 

It  is  beyond  the  province  of  this  lecture  to  enter  into 
history,  but  it  is  impossible  not  to  refer  to  the  fact  that 
the  chief  impulse  to  astronomical  work  in  this  direction 
was  given  by  Dr.  Huggins,  our  chairman  to-night;  nay, 
more,  except  for  the  early  contributions  of  Fraunhofer  to 
the  subject,  Dr.  Huggins  certainly  is  the  father  of  sidereal 
spectroscopy,  and  that  not  in  one  but  in  every  branch  of 
it.  He  has  devised  the  means,  pointed  the  way,  and,  while 
in  many  branches  of  the  work  he  still  continues  to  lead 
the  way,  he  has  of  necessity  left  the  development  of  other 
branches  to  other  hands. 

From  an  astrometer's  point  of  view  the  most  important 

9  The  older  methods  enabled  us  to  measure  motions  at  right  angles 
to  the  line  of  sight,  but  till  the  spectroscope  came  we  could  not  measure 
motions  in  the  line  of  sight. 


484  GILL 

advance  that  has  been  made  in  spectroscopy  of  recent  years 
is  the  sudden  development  of  precision  in  the  measures  of 
star  motion  in  the  line  of  sight.  The  method  remained  for 
fifteen  or  sixteen  years  quite  undeveloped  from  the  condi- 
tion in  which  it  left  the  hands  of  Dr.  Huggins,  and  cer- 
tainly no  progress  in  the  accuracy  attained  by  Dr.  Hug- 
gins  was  made  till  the  matter  was  taken  up  by  Dr.  Vogel 
at  Potsdam.  At  a  single  step  Dr.  Vogel  has  raised  the 
precision  of  the  work  from  that  of  observations  in  the  days 
of  Ptolemy  to  that  of  the  days  of  Bradley — from  the  days 
of  the  old  sights  and  pinnules  to  the  days  of  telescopes. 
Therefore  I  take  Potsdam  observation  as  the  best  type  of 
a  modern  spectroscopic  observation  for  description,  espe- 
cially as  I  have  recently  visited  Dr.  Vogel  at  Potsdam,  and 
he  has  kindly  given  me  a  photograph  of  his  spectroscope, 
as  well  as  of  some  of  the  work  done  with  it. 

The  method  of  observation  consists  simply  in  inserting 
a  small  photographic  plate  in  the  dark  slide,  directing  the 
telescope  to  the  star,  and  keeping  the  image  of  the  star 
continuously  on  the  slit  during  an  exposure  of  about  an- 
hour;  and  this  is  what  is  obtained  on  development  of  the 
picture. 

If  the  star  remained  perfectly  at  rest  between  the  jaws 
of  the  slit  the  spectrum  would  be  represented  by  a  single 
thread  of  light,  and  of  course  no  lines  would  be  visible  upon 
such  a  thread;  but  the  observer  intentionally  causes  the 
star  image  to  travel  a  little  along  the  slit  during  the  time 
of  exposure,  and  so  a  spectrum  of  sensible  width  is  ob- 
tained. 

You  will  remark  how  beautifully  sharp  are  the  faint 
lines  in  this  spectrum.  Those  who  have  tried  to  observe 
the  spectrum  of  Sirius  in  the  ordinary  way  know  that  many 
of  these  fine  lines  can  not  be  seen  or  measured  with  cer- 
tainty. The  reason  is  that  on  account  of  irregularities  in 
atmospheric  refraction,  the  image  of  a  star  in  the  telescope 
is  rarely  tranquil;  sometimes  it  shines  brightly  in  the  centre 
of  the  slit,  sometimes  barely  in  the  slit  at  all,  and  the  eye 
becomes  puzzled  and  confused.  But  the  photographic  eye 


WORK   IN  A  MODERN  OBSERVATORY  485 

is  not  in  the  least  disturbed;  when  the  star  image  is  in  the 
slit,  the  plate  goes  on  recording  what  it  sees,  and  when 
the  star  is  not  in  the  slit  the  plate  does  nothing,  and  it 
is  of  no  consequence  whatever  how  rapidly  these  alternate 
appearances  and  disappearances  recur.  The  only  differ- 
ence is  that  when  the  air  is  very  steady  and  the  star's  image, 
therefore,  always  in  the  slit,  the  exposure  takes  less  time 
than  when  the  star  is  unsteady. 

That  is  one  reason  why  the  Potsdam  results  are  accu- 
rate; and  there  are  many  other  reasons  besides,  into  which 
I  can  not  now  enter.  What,  however,  it  is  very  important 
to  note  is  this:  that  we  have  here  a  method  which  is  to  a 
great  extent  independent  of  the  atmospheric  disturbances 
which  in  all  other  departments  of  astronomical  observation 
have  imposed  a  limit  to  their  precision.  Accurate  astro- 
spectroscopy,  therefore,  may  be  pushed  to  a  degree  of  per- 
fection which  is  limited  only  by  the  optical  aid  at  our  dis- 
posal and  by  the  sensibility  of  our  photographic  plates. 

And  now  I  think  we  have  sufficiently  considered  the 
ordinary  processes  of  astronomical  observation  to  illustrate 
the  character  of  the  work  of  an  astronomer  at  night;  the 
picture  should  be  completed  by  an  account  of  his  work 
by  day.  But  to  go  into  that  matter  in  detail  would  cer- 
tainly not  be  within  the  limits  of  this  lecture.  It  is  better 
that  I  should  in  conclusion  touch  upon  some  recent  re- 
markable results  of  these  day  and  night  labours.  It  is 
these,  after  all,  that  most  appeal  to  you;  it  is  for  these  that 
the  astronomer  labours;  it  is  the  prospect  of  them  that 
lightens  the  long  watches  of  the  night  and  gives  life  to 
the  otherwise  dead  bones  of  mechanical  routine. 

Let  us  take  first  some  spectroscopic  results.  To  ex- 
plain their  meaning  let  me  remind  you  for  a  moment  of 
the  familiar  analogy  between  light  and  sound. 

The  pitch  of  a  musical  note  depends  on  the  rapidity  of 
the  vibrations  communicated  to  the  air  by  the  reed  or 
string  of  the  musical  instrument  that  produces  the  note,  a 
low  note  being  given  by  slow  vibrations  and  a  high  one 
by  quick  vibrations. 


486  GILL 

Just  in  the  same  way  red  light  depends  on  relatively 
slow  vibrations  of  ether,  and  blue  or  violet  light  on  rela- 
tively quick  vibrations.  Well,  if  there  is  a  railway  train 
rapidly  approaching  one,  and  the  engine  sounds  its  whis- 
tle, more  waves  of  sound  from  that  whistle  will  reach  the 
ear  in  a  second  of  time  than  would  reach  the  ear  were  the 
train  at  rest.  On  the  other  hand,  if  the  train  is  travelling 
at  the  same  rate  away  from  the  observer,  fewer  waves  of 
sound  will  reach  his  ears  in  a  second  of  time.  Therefore 
an  observer  beside  the  line  should  observe  a  distinct  change 
of  pitch  in  the  note  of  the  engine  whistle  as  the  train  passes 
him,  and  as  a  matter  of  fact  such  a  change  of  pitch  can  be 
and  has  been  observed. 

Just  in  the  same  way,  if  a  source  of  light  could  be 
moved  rapidly  enough  toward  an  observer  it  would  become 
bluer,  or  if  away  from  him  it  would  become  red  in  colour; 
only  it  would  require  a  change  of  velocity  in  the  moving 
light  of  some  thousands  of  miles  per  second  in  order  to 
render  the  difference  of  colour  sensible  to  the  eye.  The 
experiment  is,  therefore,  not  likely  to  be  frequently  shown 
at  this  lecture  table! 

But  the  spectroscope  enables  such  changes  of  colour 
to  be  measured  with  extreme  precision.  Here  on  the 
screen  is  the  most  splendid  illustration  of  this  that  exists 
at  present — viz.,  copies  of  three  negatives  of  the  spectrum 
of  a  Aurigae,  taken  at  Potsdam  in  October  and  December 
of  1888,  and  in  March,  1899. 

The  white  line  (the  picture  being  a  positive)  represents 
the  bright  line  Hy  given  by  the  artificial  light  of  hydrogen ; 
the  strong  black  line  in  the  picture  of  the  star  spectrum 
corresponds  to  the  black  absorption  line  which  is  due  to 
hydrogen  in  the  atmosphere  of  the  star. 

Why  is  it  that  the  artificial  hydrogen  line  does  not  cor- 
respond with  the  stellar  line  in  these  three  pictures?  The 
answer  is,  either  the  star  is  moving  toward  or  from  the 
earth  in  the  line  of  sight,  or  the  earth  is  moving  from  or 
toward  the  star.  But  in  December  the  earth  in  its  motion 
round  the  sun  is  moving  at  right  angles  to  the  direction 


WORK   IN   A   MODERN   OBSERVATORY  487 

of  a  Aurigse;  why,  then,  does  the  stellar  hydrogen  line 
not  agree  in  position  with  the  terrestrial  hydrogen  line? 
The  simple  explanation  is  that  a  Aurigse  is  moving  with 
respect  to  the  sun. 

In  what  way  is  it  moving?  Well,  that  is  also  clear;  the 
stellar  line  is  displaced  toward  the  red  end  of  the  spectrum 
— that  is  to  say,  the  star  light  is  redder  than  it  should  be  in 
consequence  of  a  motion  of  recession;  this  proves  that  the 
star  is  moving  away  from  us,  and  measures  of  the  photo- 
graph show  the  rate  of  this  motion  to  be  fifteen  and  a  half 
miles  per  second.  We  also  know  that  in  October  the  earth 
in  its  motion  round  the  sun  is  moving  toward  a  Aurigae 
nearly  at  the  same  rate  as  we  have  just  seen  that  a  Aurigae 
is  running  away  from  the  sun.  Consequently,  at  that  time, 
their  relative  motions  are  nearly  insensible,  because  both 
are  going  at  the  same  rate  in  the  same  direction,  and  we  find 
accordingly,  in  October,  that  the  positions  of  the  stellar 
and  artificial  hydrogen  lines  perfectly  correspond.  Finally, 
in  March,  the  earth  in  its  motion  round  the  sun  is  moving 
away  from  a  Aurigae,  and  as  a  Aurigae  is  also  running  away 
from  the  sun,  the  starlight  becomes  so  much  redder  than 
normal  that  the  stellar  hydrogen  line  is  shifted  completely 
to  one  side  of  the  hydrogen  and  artificial  line. 

The  accuracy  of  these  results  may  be  proved  as  follows: 

If  we  measure  all  the  photographs  of  a  Aurigae  which 
Dr.  Vogel  has  obtained,  we  can  derive  from  each  a  deter- 
mination of  the  relative  velocity  of  the  motion  of  the  star 
with  respect  to  our  earth. 

Of  course  these  velocities  are  made  up  of  the  velocity 
of  motion  of  a  Aurigae  with  respect  to  the  sun  (which  we 
may  reasonably  assume  to  be  a  uniform  velocity)  and  the 
velocity  of  the  earth  due  to  its  motion  round  the  sun.  But 
the  velocity  of  the  earth's  motion  in  its  orbit  is  known  with 
an  accuracy  of  about  one  five-hundredth  part  of  its  amount, 
and  therefore,  within  that  accuracy,  we  can  allow  precisely 
for  its  effect  on  the  relative  velocity  of  the  earth  and 
a  Aurigse.  When  we  have  done  so  we  get  the  annexed 
results  for  the  velocity  of  the  motion  of  a  Aurigae  with  re- 


488 


GILL 


spect  to  the  sun.  You  see  by  the  annexed  table  how  beau- 
tifully they  agree  in  the  Potsdam  results,  and  how  com- 
paratively rough  and  unreliable  are  the  results  obtained 
by  the  older  method  of  Greenwich.8 

I  believe  that  in  a  few  years — at  least  in  a  period  of 
time  that  one  may  hope  to  see — we  shall  not  be  content 
merely  to  correct  our  results  for  the  motion  of  the  earth 
in  its  orbit  only,  and  so  test  our  observations  of  motion 

9  a  Auriga — Potsdam 


DATS. 

Observed  relative 
motion  of  earth 
and  star.    Miles 
per  second. 

Motion  of  earth. 

Concluded  motion. 
Star  relative  to  sun. 

1888. 

+  2.5 

—  13.0 

+  15.5 

+  3-  r 

—  12.4 

+  15.5 

-T-3-I 

—  12.4 

+  15.5 

"       28th                   .    .    » 

+  2.5 

—  ii.  8 

+  14.3 

November  oth            

+  6.8 

—8.7 

+  15.5 

+  n.  8 

—3.  i 

+  14.9 

"          i3th              .    ... 

+  14..  Q 

+  0.6 

+  I4-3 

1889. 

+  2O.5 

+  0.8 

+  1^7 

+  32.0 

+  14.  a 

+  18.6 

+  34.2 

+  16.8 

+  17.4 

a  Auriga — Greenwich 


DATB. 

Observed  relative 
motion  of  earth 
and  star.   Miles 
per  second. 

Motion  of  earth. 

Concluded  motion. 
Star  relative  to  sun. 

1887. 
January   26th                       .  . 

+  16.4 

+  12   6 

+-1   8 

February  i6th     

+  ^4-4 

+  11    Q 

+•18  5 

+  SQ.8 

—  I*}.  5 

+  52.  ^ 

"       25th 

+  25.4 

—  T7    O 

+  ^8    4 

"          2Qth    

+  40.6 

—  12.  1 

+  52    7 

1888. 

+  2Q.O 

—  1.2 

+  16.2 

l88q. 

February  i$th     

+  2^.8 

+  16  o 

+  7   8 

March  5th  

+    2O.^ 

+  17.  1 

+•1    2 

September  I7th 

+  18  6 

—  jo    <i 

+  *?'?    7 

"          igth    

+  21.8 

—  16  7 

+  •^8  5 

"          25th    . 

+  24  8 

—  16  5 

+  AT    3 

November  25th  .  , 

+  24.5 

—  4..O 

+  20.4 

WORK   IN   A   MODERN   OBSERVATORY  489 

in  the  line  of  sight,  but  that  we  shall  have  arrived  at  a 
certainty  and  precision  of  working  which  will  permit  the 
process  to  be  reversed,  and  that  we  shall  be  employing  the 
spectroscope  to  determine  the  velocity  of  the  earth's  mo- 
tion in  its  orbit,  or,  in  other  words,  to  determine  the 
fundamental  unit  of  astronomy,  the  distance  of  the  sun 
from  the  earth. 

I  will  take  as  another  example  one  recent  remarkable 
spectroscopic  discovery.  Miss  Murray,  in  examining  a 
number  of  photographs  of  stellar  spectra  taken  at  Harvard 
University,  discovered  that  in  every  spectrum  of  ft  Aurigae 
certain  lines  doubled  themselves  every  two  days,  becoming 
single  in  the  intermediate  days.  Accurate  Potsdam  obser- 
vations confirmed  the  conclusion. 

The  picture  on  the  screen  shows  the  spectrum  of 
P  Aurigse  photographed  on  November  226.  and  25th  of  last 
year.  In  the  first  the  lines  are  single,  in  the  other  every 
line  is  doubled.  Measures  and  discussions  of  a  number  of 
these  photographs  have  shown  that  the  doubling  of  the 
lines  is  perfectly  accounted  for  by  the  supposition  of  two 
suns  revolving  round  each  other  in  a  period  of  four  days, 
each  moving  at  a  velocity  of  about  seventy  miles  a  second 
in  its  orbit. 

When  one  star  is  approaching  us  and  the  other  reced- 
ing, the  lines  in  the  spectrum  formed  by  the  light  of  the  first 
star  will  be  moved  toward  the  blue  end  of  the  spectrum, 
those  in  the  spectrum  of  the  second  star  toward  the  red 
end  of  the  spectrum;  then,  as  the  two  stars  come  into  the 
same  line  with  us,  their  motions  become  at  right  angles  to 
the  line  of  sight,  and  their  two  spectra,  not  being  affected 
by  motion,  will  perfectly  coincide;  but  then,  after  the  stars 
cross,  their  spectra  again  separate  in  the  opposite  direction, 
and  so  they  go  on. 

Thus  by  means  of  their  spectra  we  are  in  a  position  to 
watch  and  to  measure  the  relative  motions  of  two  objects 
that  we  can  never  see  apart;  nay,  more,  we  can  determine 
not  only  their  period  of  revolution,  but  also  the  velocity 
of  their  motions  in  their  orbits.  Now,  if  we  know  the  time 


490  GILL 

that  a  body  takes  to  complete  its  revolution,  and  the 
velocity  at  which  it  moves,  clearly  we  know  the  dimensions 
of  the  orbit;  and  if  we  know  the  dimensions  of  an  orbit 
we  know  what  attractive  force  is  necessary  to  compel  the 
body  to  keep  in  that  orbit,  and  thus  we  are  able  to  weigh 
these  bodies.  The  components  of  ft  Aurigse  are  two  suns, 
which  revolve  about  each  other  in  four  days;  they  are  only 
between  7,000,000  and  8,000,000  miles  (or  one  twelfth  of 
our  distance  from  the  sun)  apart,  and  if  they  are  of  equal 
weight  they  each  weigh  rather  over  double  the  weight  of 
the  sun. 

I  have  little  doubt  that  these  facts  do  not  represent  a 
permanent  condition,  but  simply  a  stage  of  evolution  in  the 
life  history  of  the  system,  an  earlier  stage  of  which  may 
have  been  a  nebular  one. 

Other  similar  double  stars  have  been  discovered,  both 
at  Potsdam  and  at  Cambridge,  United  States — stars  that 
we  shall  never  see  separately  with  the  eye  aided  by  the  most 
powerful  telescope;  but  time  does  not  permit  me  to  enter 
into  any  account  of  them. 

I  pass  now  to  another  recent  result  that  is  of  great 
cosmical  interest.  The  Cape  photographic  star  charting 
of  the  southern  hemisphere  has  been  already  referred  to. 
In  comparing  existing  eye  estimates  of  magnitude  by  Dr. 
Gould  with  the  photographic  determinations  of  these  mag- 
nitudes, both  Professor  Kapteyn  and  myself  have  been 
greatly  struck  with  a  very  considerable  systematic  discord- 
ance between  the  two.  In  the  rich  parts  of  the  sky — that 
is,  in  the  Milky  Way — the  stars  are  systematically  photo- 
graphically brighter  by  comparison  with  the  eye  observa- 
tions than  they  are  in  the  poorer  part  of  the  sky,  and  that 
not  by  any  doubtful  amount  but  by  half  or  three  fourths 
of  a  magnitude.  One  of  two  things  was  certain:  either 
that  the  eye  observations  were  wrong,  or  that  the  stars  of 
the  Milky  Way  are  bluer  or  whiter  than  other  stars.  But 
Professor  Pickering,  of  Cambridge,  America,  has  lately 
been  making  a  complete  photographic  review  of  the 
heavens,  and  by  placing  a  prism  in  front  of  the  telescope 


WORK   IN   A   MODERN   OBSERVATORY  491 

he  has  made  pictures  of  the  whole  sky  like  this.  (Here  two 
examples  of  plates  of  Pickering's  spectroscopic  "  Durch- 
musterung  "  were  exhibited  on  the  screen.)  He  has  dis- 
cussed the  various  types  of  the  spectra  of  the  brighter  stars 
as  thus  revealed,  according  to  their  distribution  in  the  sky. 
He  finds  thus  that  the  stars  of  the  Sirius  type  occur  chiefly 
in  the  Milky  Way,  while  stars  of  other  types  are  fairly 
divided  over  the  sky. 

Now,  stars  of  the  Sirius  type  are  very  white  stars,  very 
rich  relative  to  other  stars  in  the  rays  which  act  most 
strongly  on  a  photographic  plate.  Here,  then,  is  the  ex- 
planation of  the  results  of  our  photographic  star-charting, 
and  of  the  discordance  between  the  photographic  and  visual 
magnitudes  in  the  Milky  Way. 

The  results  of  the  Cape  charting  further  show  that  it  is 
not  alone  to  the  brighter  stars  that  this  discordance  ex- 
tends, but  it  extends  also,  though  in  a  rather  less  degree, 
to  the  fainter  stars  of  the  Milky  Way.  Therefore  we  may 
come  to  the  very  remarkable  conclusion  that  the  Milky 
Way  is  a  thing  apart,  and  that  it  has  been  developed  perhaps 
in  a  different  manner,  or  more  probably  at  a  different  and 
probably  later  epoch  from  the  rest  of  the  sidereal  universe. 

Here  is  another  interesting  cosmical  revelation  which 
we  owe  to  photography.  You  all  know  the  beautiful  con- 
stellation Orion,  and  many  in  this  theatre  have  before  seen 
the  photograph  of  the  nebula  which  is  now  on  the  screen, 
taken  by  Mr.  Roberts.  Here  is  another  photograph  of  the 
same  object  taken  with  a  much  longer  exposure.  You  see 
how  over-exposed — in  fact,  burned  out — the  brightest  part 
of  the  picture  is,  and  yet  what  a  wonderful  development  of 
faint  additional  nebulous  matter  is  revealed.  But  I  do 
not  think  that  many  persons  in  this  room  have  seen  this 
picture,  and  probably  very  few  have  any  idea  what  it  repre- 
sents. It  is  from  the  original  negative  taken  by  Professor 
Pickering,  with  a  small  photographic  lens  of  short  focus, 
after  six  hours'  exposure  in  the  clear  air  of  the  Andes, 
10,000  feet  above  sea  level.  The  field  embraces  the  three 
well-known  stars  in  the  belt  of  Orion  on  the  one  hand, 


4Q2  GILL 

and  fi  Orionis  (Rigel)  on  the  other.  You  can  hardly  rec- 
ognise these  great  white  patches  as  stars;  their  ill-defined 
character  is  simply  the  result  of  excessive  over-exposure. 
But  mark  the  wonders  which  this  long  exposure  with  a 
lens  of  high  intrinsic  brilliancy  of  image  has  revealed.  Here 
is  the  great  nebula,  of  course  terribly  over-exposed,  but 
note  its  wonderful  fainter  ramifications.  See  how  the  whole 
area  is  more  or  less  nebulous,  and  surrounded,  as  it  were, 
with  a  ring  fence  of  nebulous  matter.  This  nebulosity 
shows  a  special  concentration  about  y9  Orionis. 

Well,  when  Professor  Pickering  got  this  wonderful  pic- 
ture, knowing  that  I  was  occupied  with  investigations  on 
the  distance  of  the  fixed  stars,  he  wrote  to  ask  whether 
I  had  made  any  observations  to  determine  the  distance  of 
&  Orionis,  as  it  would  be  of  great  interest  to  know  from 
independent  evidence  whether  this  very  bright  star  was 
really  near  to  us  or  not.  It  so  happens  that  the  observa- 
tions were  made,  and  their  definitive  reduction  has  shown 
that  0  Orionis  is  really  at  the  same  distance  from  us  as  are 
the  faint  comparison  stars.  0  Orionis  is,  therefore,  prob- 
ably part  and  parcel  of  an  enormous  system  in  an  ad- 
vanced but  incomplete  state  of  stellar  evolution,  and  that 
what  we  have  seen  in  this  wonderful  picture  is  all  a  part 
of  that  system. 

I  should  explain  what  I  mean  by  an  elementary  or  by 
an  advanced  state  of  stellar  evolution.  There  is  but  one 
theory  of  celestial  evolution  which  has  so  far  survived  the 
test  of  time  and  comparison  with  observed  facts — viz.,  the 
nebular  hypothesis  of  Laplace.  Laplace  supposed  that  the 
sun  was  originally  a  huge  gaseous  or  nebulous  mass  of  a 
diameter  far  greater  than  the  orbit  of  Neptune.  I  say 
originally;  do  not  misunderstand  me.  We  have  finite 
minds;  we  can  imagine  a  condition  of  things  which  might 
be  supposed  to  occur  at  any  particular  instant  of  time, 
however  remote,  and  at  any  particular  distance  of  space, 
however  great;  and  we  may  frame  a  theory  beginning  at 
another  time  still  more  remote,  and  so  on.  But  we  can 
never  imagine  a  theory  beginning  at  an  infinite  distance 


WORK   IN   A   MODERN   OBSERVATORY  493 

of  time  or  at  an  infinitely  distant  point  in  space.  Thus, 
in  any  theory  which  man  with  his  finite  mind  can  devise, 
when  we  talk  of  "  originally  "  we  simply  mean  at  or  during 
the  time  considered  in  our  theory. 

Now,  Laplace's  theory  begins  at  a  time,  millions  on 
millions  of  years  ago,  when  the  sun  had  so  far  disentangled 
itself  from  chaos,  and  its  component  gaseous  particles  had 
by  mutual  attraction  so  far  coalesced  as  to  form  an  enor- 
mous gaseous  ball,  far  greater  in  diameter  than  the  orbit 
of  the  remotest  planet  of  our  present  system.  The  central 
part  of  this  ball  revolved.  There  is  nothing  improbable  in 
this  hypothesis.  If  gaseous  matter  came  together  from 
different  parts  of  space  such  coalition  would  unquestion- 
ably occur,  and  as  in  the  meeting  of  opposite  streams  of 
water  or  of  opposite  currents  of  wind,  vortices  would  be 
created  and  revolution  about  an  axis  set  up,  such  as  we 
are  familiar  with  in  the  case  of  whirlpools  or  cyclones. 
The  resultant  would  be  rotation  of  the  whole  globular 
gaseous  mass  about  an  axis. 

Now,  this  gaseous  globe  begins  to  cool,  and  as  it  cools 
it  necessarily  contracts.  Then  follows  a  necessary  result 
of  contraction — viz.,  the  rotation  becomes  more  rapid. 
This  is  a  well-known  fact  in  dynamics,  about  which  there 
is  no  doubt.  Thus,  the  cooling  and  the  contracting  go  on, 
and  simultaneously  the  velocity  of  rotation  becomes  greater 
and  greater.  At  last  the  time  arrives  when,  for  the  outside 
particles,  the  velocity  of  rotation  becomes  such  that  the 
centrifugal  force  is  greater  than  the  attractive  force,  and 
so  the  outside  particles  break  off  and  form  a  ring.  Then, 
as  the  process  of  cooling  and  contraction  proceed  still 
further,  another  ring  is  formed,  and  so  on  until  we  have 
finally  a  succession  of  rings  and  a  condensed  central  ball. 
If  from  any  cause  the  cooling  of  any  of  these  rings  does 
not  go  on  uniformly,  or  if  some  of  the  gaseous  matter  of 
the  ring  is  more  easily  liquefied  than  others,  then  probably 
a  simple  nucleus  of  liquid  matter  will  be  formed  in  that 
ring,  and  this  nucleus  will  finally  by  attraction  absorb  the 
whole  of  the  matter  of  which  the  ring  is  composed — at 


494 


GILL 


first  as  a  gaseous  ball  with  a  condensed  nucleus,  and  tfiis 
will  finally  solidify  into  a  planet.  Or,  meanwhile,  this  yet 
unformed  planet  may  repeat  the  history  of  its  parent  sun. 
By  contraction,  and  consequent  acceleration  of  its  rota- 
tion, it  may  throw  off  one  or  more  rings,  which  in  like 
manner  condense  into  satellites  like  our  moon,  or  those  of 
Jupiter,  Saturn,  Uranus,  or  Neptune.  Such,  very  briefly 
outlined,  is  the  celebrated  nebular  hypothesis  of  Laplace. 
No  one  can  positively  say  that  the  hypothesis  is  true;  still 
less  can  any  one  say  that  it  is  untrue.  Time  does  not 
permit  me  to  enter  into  the  very  strong  proofs  which 
Laplace  urged  in  favour  of  its  acceptance. 

But  I  beg  you  for  one  moment  to  cast  your  imagina- 
tion back  to  a  period  of  time  long  antecedent  to  that  when 
our  sun  had  begun  to  disentangle  itself  from  chaos,  and 
when  the  fleecy  clouds  of  cosmic  stuff  had  but  begun 
to  rush  together.  What  should  we  see  in  such  a  case  were 
there  a  true  basis  for  the  theory  of  Laplace?  Certainly, 
in  the  first  place,  we  should  have  a  huge  whirlpool  or 
cyclone  of  cosmic  gaseous  stuff,  the  formation  of  rings, 
and  the  condensation  of  these  rings  into  gaseous  globes. 

Remembering  this,  look  now  on  this  wonderful  photo- 
graph of  the  nebula  in  Andromeda,  made  by  Mr.  Roberts. 
In  the  largest  telescopes  this  nebula  appears  simply  as  an 
oval  patch  of  nearly  uniform  light,  with  a  few  dark  canals 
through  it,  but  no  idea  of  its  true  form  can  be  obtained, 
no  trace  can  be  found  of  the  significant  story  which  this 
photograph  tells.  It  is  a  picture  that  no  human  eye  un- 
aided by  photography  has  ever  seen.  It  is  a  true  picture, 
drawn  without  the  intervention  of  the  hand  of  infallible 
man,  and  uninfluenced  by  his  bias  or  imagination.  Have 
we  not  here,  so  at  least  it  seems  to  me,  a  picture  of  a 
very  early  stage  in  the  evolution  of  a  star  cluster  or  sun- 
system — a  phase  in  the  history  of  another  star-system  simi- 
lar to  that  which  once  occurred  in  our  own — millions  and 
millions  of  years  ago — when  our  earth — nay,  even  our  sun 
itself — "  was  without  form  and  void,"  and  "  darkness  was 
on  the  face  of  the  deep  "  ? 


WORK   IN   A  MODERN  OBSERVATORY  495 

During  this  lecture  I  have  been  able  to  trace  but  very 
imperfectly  the  bare  outlines  of  an  astronomer's  work  in 
a  modern  observatory,  and  to  give  you  a  very  few  of  its 
latest  results — results  which  do  not  come  by  chance,  but 
by  hard  labour,  and  to  men  who  have  patience  to  face  dull, 
daily  routine  for  the  love  of  science — to  men  who  realize 
the  imperfections  of  their  methods,  and  are  constantly  on 
the  alert  to  improve  them. 

The  mills  of  the  astronomer  grind  slowly,  and  he  must 
be  infinitely  careful  and  watchful  if  he  would  have  them, 
like  the  mills  of  God,  to  grind  exceedingly  small.  I  think 
he  may  well  take  for  his  motto  these  beautiful  lines: 

"  Like  the  star 
Which  shines  afar, 
Without  haste, 
Without  rest, 
Let  each  man  wheel 
With  steady  sway 
Round  the  task 
Which  rules  the  day, 
And  do  his  best." 


THE  SYSTEM  OF  THE  WORLD— 
THE  NEBULAR  HYPOTHESIS 


BY 

PIERRE   SIMON,   MARQUIS   DE   LAPLACE 
(Born  1749  J  died  1827) 


THE   SYSTEM   OF  THE  WORLD1 

WE  have  five  phenomena,  as  follows,  from  which 
to  ascend  to  the  cause  of  the  primitive  motions 
of  the  planetary  system:  The  motions  of  the 
planets  are  in  the  same  direction  and  almost  in  the  same 
plane;  the  motions  of  the  satellites  are  in  the  same  direc- 
tion as  the  planets;  the  motions  of  rotation  of  these  sev- 
eral bodies  and  of  the  sun  are  in  the  same  direction  as 
their  motions  of  translation,  and  in  planes  only  slightly 
inclined  to  each  other;  the  eccentricity  of  the  orbits  of  the 
planets  and  of  the  satellites  is  very  small;  and,  lastly,  the 
eccentricity  of  the  orbits  of  the  comets  is  large,  although 
the  inclinations  of  these  bodies  may  have  been  the  result  of 
chance. 

Buffon  is  the  only  philosopher  known  to  me  who,  since 
the  discovery  of  the  true  system  of  the  world,2  has  at- 
tempted to  explain  the  origin  of  the  planets  and  of  the 
satellites.  He  supposes  that  a  comet,  falling  upon  the 
sun,  has  expelled  from  it  a  torrent  of  matter  which  has 
coalesced  once  more,  at  a  distance,  into  various  globes, 
larger  or  smaller,  and  nearer  or  farther  from  that  heav- 
enly body;  and  that  these  globes,  having  become  opaque 
and  solid  as  they  cooled,  are  the  planets  and  their  sat- 
ellites. 

This  hypothesis  explains  the  first  of  the  five  phenomena 

1 "  (Euvres  Completes  de  Laplace  "  (reprint  of  1884,  from  fifth  edi- 
tion, 1835),  publiees  sous  les  auspices  de  1' Academic  des  Sciences  par 
MM.  les  Secretaires  perpetuels.  Paris,  1884,  vol.  vi.  "  Systeme  du 
Monde,"  note  vii,  pp.  498-509.  Translated  by  SARA  CARR  UPTON. 

1  By  Copernicus,  Kepler,  and  Newton. — EDITOR. 

499 


5oo 


LAPLACE 


previously  cited,-  for  it  is  clear  that  all  the  bodies  thus 
formed  must  move  very  nearly  in  the  plane  which  passes 
through  the  centre  of  the  sun,  and  in  the  direction  of  the 
torrent  of  matter  which  has  produced  them;  the  other  four 
phenomena  seem  to  me  inexplicable  by  his  method.  In 
truth,  the  absolute  motions  of  the  molecules  of  a  planet 
would  then  be  in  the  same  direction  as  the  motion  of  its 
centre  of  gravity.  But  it  does  not  at  all  follow  that  the 
rotation-motion  of  the  planet  would  have  the  same  direc- 
tion; thus  the  earth  might  turn  from  east  to  west,  and 
yet  the  absolute  motion  of  each  of  its  molecules  might  be 
directed  from  west  to  east.  This  would  also  apply  to  the 
motion  of  revolution  of  the  satellites,  the  direction  of  which, 
under  the  hypothesis  in  question,  is  not  necessarily  the 
same  as  the  direction  of  the  motion  of  translation  of  the 
planets  in  their  orbits. 

A  phenomenon  not  only  very  difficult  to  explain  under 
this  hypothesis,  but  one  even  contrary  to  it,  is  the  small 
eccentricity  of  the  planetary  orbits.  It  is  known  from  the 
theory  of  central  forces  that  if  a  body  moving  in  an  orbit 
revolving  around  the  sun  touches  the  surface  of  that 
heavenly  body,  it  will  constantly  return  to  the  same  point 
there  at  each  of  its  revolutions;  whence  it  follows  that,  had 
the  planets  been  detached  from  the  sun  at  their  origin,  they 
would  touch  it  upon  each  of  their  returns  toward  that 
body,  and  that  their  orbits,  far  from  being  circular,  would 
be  very  eccentric.  It  is  true  that  a  torrent  of  matter  ex- 
pelled from  the  sun  can  not  be  exactly  compared  to  a 
globe  which  grazes  its  surface;  the  impulsion  which  the 
parts  of  such  a  torrent  would  receive  from  each  other,  and 
the  reciprocal  attraction  they  would  exert  among  them- 
selves as  they  changed  their  direction  of  motions,  might 
force  their  perihelions  farther  from  the  sun.  But  their 
orbits  would  always  be  very  eccentric,  or  at  least  they 
could  not  all  of  them  have  small  eccentricities,  except  by 
the  most  extraordinary  chance.  Lastly,  under  Buffon's 
hypothesis  it  can  not  be  explained  why  the  orbits  of  more 
than  one  hundred  comets  already  observed  are  all  much 


THE   SYSTEM   OF  THE  WORLD 


501 


elongated.  This  hypothesis,  therefore,  is  very  far  from  ex- 
plaining the  preceding  phenomena.  Let  us  see  if  it  is  pos- 
sible to  rise  to  their  veritable  cause. 

Whatever  may  be  the  nature  of  this  cause,  since  it 
has  produced  or  directed  the  motions  of  the  planets  it  must 
have  comprehended  all  such  bodies,  and  in  view  of  the 
prodigious  distances  which  separate  them  one  from  an- 
other, it  can  only  have  been  a  fluid  of  immense  extension. 
To  have  given  the  man  almost  circular  motion  in  the 
same  direction  around  the  sun,  this  fluid  must  have  sur- 
rounded that  body  like  an  atmosphere.  The  considera- 
tion of  the  planetary  motions  thus  leads  us  to  think 
that  by  reason  of  excessive  heat  the  atmosphere  of  the 
sun  originally  extended  beyond  the  orbits  of  all  the 
planets,  and  that  it  has  successively  shrunk  to  its  pres- 
ent limits. 

In  the  primitive  condition  that  we  have  ascribed  to  the 
sun  it  resembled  the  nebulae,  which  the  telescope  shows 
to  be  composed  of  a  nucleus  more  or  less  brilliant,  sur- 
rounded by  nebulosity  which,  as  it  condenses  on  the  sur- 
face of  the  nucleus,  transforms  it  into  a  star.  If,  by  analogy, 
we  conceive  all  stars  to  be  formed  in  this  manner,  we  may 
imagine  their  former  state  of  nebulosity  which  was  itself 
preceded  by  other  states  in  which  the  nebulous  matter  was 
still  more  diffused,  the  nucleus  still  less  luminous.  We  thus 
reach,  going  backward  as  far  as  possible,  a  nebulosity  so 
diffused  that  its  existence  is  barely  manifest. 

For  a  long  time  the  special  configuration  of  certain 
stars  visible  to  the  naked  eye  has  struck  philosophical  ob- 
servers. Mitchel  has  already  remarked  how  improbable  it 
is  that  the  stars  of  the  Pleiades,  for  example,  should  have 
been  clustered  together  in  the  small  region  containing 
them  as  the  result  of  mere  chance,  and  he  concludes  there- 
from that  this  group  of  stars  and  other  similar  groups  dis- 
played before  us  in  the  sky  are  the  effects  of  some  primi- 
tive cause,  or  of  a  general  law  of  Nature.  Such  groups 
are  a  necessary  result  of  the  condensation  of  nebulae  with 
several  nuclei,  for  it  is  plain  that  the  nebulous  matter,  being 


502  LAPLACE 

constantly  attracted  by  these  different  nuclei,  must  finally 
form  a  group  of  stars  like  the  Pleiades.  The  condensation 
of  nebulae  with  two  nuclei  similarly  will  form  stars  in  very 
close  proximity,  which  will  turn  one  around  the  other, 
similar  to  those  double  stars  whose  relative  motions  have 
already  been  determined. 

But  how  did  the  solar  atmosphere  determine  the  mo- 
tions of  rotation  and  revolution  of  the  planets  and  of  the 
satellites?  Had  those  bodies  penetrated  far  into  that  at- 
mosphere its  resistance  would  have  caused  them  to  fall 
upon  the  sun.  It  may  therefore  be  conjectured  that  the 
planets  were  formed  at  the  limits  of  the  solar  atmosphere 
by  the  condensation  of  the  zones  of  vapours,  which,  as  it 
cooled,  the  atmosphere  successively  abandoned  in  the  plane 
of  its  equator. 

The  atmosphere  of  the  sun  can  not  extend  outward 
indefinitely;  its  limit  is  the  point  where  the  centrifugal 
force  due  to  its  motion  of  rotation  balances  its  weight. 
Now,  as  the  cooling  contracts  the  atmosphere  by  de- 
grees, and  condenses  the  neighbouring  molecules  on  the 
surface  of  the  body,  the  motion  of  rotation  augments; 
for,  according  to  the  principle  of  "  conservation  of  areas," 
the  sum  of  the  areas  described  by  the  radius-vector  of 
each  molecule  of  the  sun  and  of  its  atmosphere,  pro- 
jected upon  the  plane  of  its  equator,  being  always  the 
same,  the  rotation  must  be  more  rapid  when  such  mole- 
cules are  near  the  sun's  centre.  The  centrifugal  force  due 
to  this  motion  thus  becoming  greater,  the  point  where  the 
weight  is  equal  to  it  is  nearer  to  that  centre.  Supposing, 
therefore,  which  it  is  natural  to  admit,  that  the  atmosphere 
reached  at  some  epoch  to  its  extreme  limit,  it  must,  in 
cooling,  have  abandoned  the  molecules  situated  at  that 
limit  and  at  the  successive  limits  due  to  the  increase  in 
the  rotation  of  the  sun.  These  abandoned  molecules  have 
continued  to  circulate  around  that  body,  since  their  centrif- 
ugal force  was  balanced  by  their  weight.  But  this  equality 
not  existing  for  the  atmospheric  molecules  situated  upon 
parallels  of  the  solar  equator,  the  latter  molecules  have 


THE   SYSTEM   OF   THE   WORLD  503 

been  drawn  by  their  weight  nearer  to  the  solar  atmosphere 
as  it  progressively  condensed,  and  they  have  not  ceased  to 
belong  to  it,  until  by  such  motion  they  have  drawn  close 
to  that  equator. 

Let  us  consider  now  the  zones  of  vapours  that  have 
been  successively  abandoned.  Such  zones  must,  in  all 
probability,  have  formed,  by  their  condensation  and  the 
mutual  attraction  of  their  molecules,  various  concentric 
rings  of  vapours  revolving  around  the  sun.  The  mutual 
friction  of  the  molecules  of  each  ring  must  have  accelerated 
some  and  retarded  others  until  all  acquired  a  like  angular 
motion.  Thus  the  real  velocities  of  the  molecules  farthest 
from  the  centre  of  the  sun  were  the  greatest.  The  follow- 
ing cause  must  also  have  contributed  to  this  difference  of 
velocity:  The  molecules  farthest  from  the  sun,  and  those 
which  from  the  effects  of  cooling  and  condensation  have 
drawn  near  the  sun  to  form  the  upper  part3  of  the  ring,  con- 
stantly described  areas  proportional  to  the  time,  since  the 
central  force  which  animated  them  was  constantly  directed 
toward  the  sun.  Now,  this  conservation  of  areas  necessi- 
tates an  increase  of  velocity  in  proportion  to  their  prox- 
imity to  the  sun.  We  see  that  the  same  cause  must  have 
diminished  the  velocity  of  the  molecules  which  have  moved 
to  form  its  lower  part  of  the  ring.4 

If  all  the  molecules  of  a  ring  of  vapours  continued  to 
condense  without  disuniting,  they  would  finally  form  a 
liquid  or  solid  ring.  But  the  regularity  in  all  the  portions 
of  the  ring  and  in  their  cooling,  demanded  by  such  a  forma- 
tion, must  have  rendered  such  a  phenomenon  extremely 
rare.  Thus  the  solar  system  presents  but  a  single  example 
of  it — the  rings  of  Saturn.  Almost  always  each  ring  of 
vapours  must  have  broken  into  several  masses  which,  mov- 
ing by  very  slightly  differing  velocities,  continued  to  re- 
volve at  the  same  distance  around  the  sun.  Such  masses 
would  have  assumed  a  spheroidal  form,  with  a  motion  of 
rotation  in  the  same  direction  as  that  of  their  revolution, 

8  The  part  most  distant  from  the  sun. 
4  That  is,  the  part  nearest  to  the  sun. 


504 


LAPLACE 


since  their  lower5  molecules  had  less  real  velocity  than 
their  upper6  ones;  they  have  found  thus  so  many  planets 
in  a  vaporous  state.  But  if  one  has  been  powerful  enough 
to  reunite  successively  by  its  attraction  all  the  others 
around  its  own  centre,  the  vaporous  ring  would  have  been 
thus  transformed  into  a  single  spheroidal  mass  of  vapours, 
revolving  around  the  sun  with  a  rotation  in  the  direction 
of  its  revolution.  The  latter  case  has  been  the  most  com- 
mon; nevertheless,  the  solar  system  presents  the  first  case 
in  the  four  small  planets  which  move  between  Jupiter  and 
Mars,7  unless  we  suppose  with  M.  Olbers  that  they  origi- 
nally formed  a  single  planet  that  has  been  divided  by 
some  great  explosion  into  several  portions  having  differ- 
ent velocities. 

Now,  if  we  follow  the  changes  that  subsequent  cooling 
would  have  produced  in  the  planets  in  a  vaporous  state, 
the  formation  of  which  we  have  been  conceiving,  we  shall 
see  produced  in  the  centre  of  each  of  them  a  nucleus  that 
will  constantly  increase  by  the  condensation  of  the  atmos- 
phere surrounding  it.  In  this  state  the  planet  would  ex- 
actly resemble  the  sun  in  the  nebulous  state  in  which  we 
have  just  been  considering  it:  the  process  of  cooling  would 
therefore  have  produced  at  the  various  limits  of  its  atmos- 
phere phenomena  similar  to  those  which  we  have  described 
— that  is  to  say,  rings  and  satellites  revolving  around  its 
centre  in  the  direction  of  its  motion  of  rotation  and  turn- 
ing in  the  same  direction  upon  themselves.  The  regular 
distribution  of  the  mass  of  the  rings  of  Saturn  around  its 
centre,  and  in  the  plane  of  its  equator,  follows  naturally 
from  this  hypothesis  and  without  it  is  inexplicable;  these 
rings  seem  to  me  to  be  ever-existing  proofs  of  the  original 
extension  of  the  atmosphere  of  Saturn  and  of  its  successive 
recessions.  Thus  the  singular  phenomena  of  the  slight  ec- 
centricity of  the  orbits  of  the  planets  and  of  the  satellites, 
of  the  slight  inclination  of  those  orbits  to  the  solar 
equator,  and  of  the  identity  of  direction  of  the  motions  of 
rotation  and  of  revolution  of  all  those  bodies  with  that  of 
8  Nearer  the  sun.  *  Farther  from  the  sun.  T  The  asteroids. 


THE   SYSTEM   OF   THE   WORLD 


505 


the  rotation  of  the  sun,  flow  from  the  hypothesis  which  we 
propose,  and  give  it  a  great  probability,  which  may  be  still 
more  augmented  by  the  following  consideration: 

All  the  satellites  that  revolve  around  a  planet  having 
been  formed,  according  to  this  hypothesis,  by  the  zones 
successively  abandoned  by  the  planet's  atmosphere,  and  its 
motion  of  rotation  having  become  more  and  more  rapid, 
the  duration  of  its  motion  of  rotation  must  be  less  than  the 
durations  of  the  revolutions  of  these  various  bodies.  The 
case  is  the  same  for  the  sun  as  compared  with  the  planets.8 
All  this  is  confirmed  by  observations.  The  duration  of  the 
revolution  of  Saturn's  nearest  ring  is,  according  to  Her- 
schel's  observations,  -^nnr  of  a  day,  and  that  of  the  rotation 
of  Saturn  is  but  •£$?$.  The  difference,  yHir>  is  very  slight, 
as  it  should  be,  because  the  portion  of  Saturn's  atmosphere 
which  the  diminution  of  heat  has  deposited  on  its  surface 
since  the  formation  of  the  ring  having  been  small,  and 
coming  from  a  small  distance  above  the  planet,  it  could 
have  only  augmented  the  rotation  of  the  planet  in  a  small 
degree. 

Had  the  solar  system  been  formed  with  perfect  regu- 
larity, the  orbits  of  the  bodies  which  compose  it  would  have 
been  circles  whose  planes,  as  well  as  the  planes  of  the  sev- 
eral equators  and  rings,  would  have  coincided  with  the 
plane  of  the  solar  equator.  But  it  is  readily  conceivable 
that  the  numberless  varieties  of  temperature  and  of  den- 
sity of  the  various  parts,  which  must  have  existed  in  these 
great  masses,  have  produced  the  eccentricities  in  their  or- 
bits and  the  deviations  in  their  motions  from  the  plane  of 
that  equator. 

*  Kepler,  in  his  work,  "  De  motibus  stellae  Martis,"  has  explained 
the  motion  of  all  the  planets  in  a  like  direction,  as  caused  by  imma- 
terial substances  emanating  from  the  surface  of  the  sun,  which,  pre- 
serving the  rotation  motion  that  they  had  at  the  surface,  gave  like  mo- 
tions to  the  planets.  He  concludes  therefrom  that  the  sun  turns  upon 
itself  in  a  period  less  than  that  of  the  revolution  of  Mercury,  which 
Galileo  determined  to  be  the  case  by  observation  soon  afterward.  Kep- 
ler's hypothesis  is  doubtless  inadmissible,  but  it  is  noteworthy  that  he 
should  have  made  the  identity  of  direction  of  the  planetary  motions  de- 
pend on  this  rotation  of  the  sun,  so  natural  would  such  tendency  appear. 


5o6  LAPLACE 

Under  our  hypothesis,  comets  are  foreign  to  the  plan- 
etary system.  In  considering  them,  as  we  have  done,  as 
small  nebulae  wandering  about  from  solar  system  to  solar 
system,  and  formed  by  the  condensation  of  the  nebulous 
matter  scattered  with  so  much  profusion  throughout  the 
universe,  we  see  that  when  they  arrive  in  that  part  of  space 
where  the  attraction  of  the  sun  predominates,  it  forces 
them  to  describe  elliptical  or  hyperbolical  orbits.  But  their 
velocities  being  equally  possible  in  all  directions,  they  must 
move  indifferently  in  all  directions  and  with  every  degree 
of  inclination  to  the  ecliptic,  which  conforms  with  what  is 
actually  observed.  Thus  the  condensation  of  the  nebulous 
matter  by  which  we  have  just  explained  the  motions  of 
rotation  and  of  revolution  of  the  planets  and  satellites  in 
the  same  direction  and  in  slightly  differing  planes,  explains 
equally  why  the  motions  of  the  comets  depart  from  the 
general  law. 

The  great  eccentricity  of  the  cometary  orbits  is  also  a 
result  of  our  hypothesis.  If  these  orbits  are  elliptical  they 
are  very  elongated,  since  their  major  axes  are  at  least  equal 
to  the  radius  of  the  sphere  of  activity  of  the  sun.  But 
these  orbits  may  be  hyperbolic,  and  if  the  axes  of  these 
hyperbolas  are  not  very  large  in  relation  to  the  mean  dis- 
tance of  the  sun  from  the  earth,  the  motion  of  the  comets 
which  describe  them  will  appear  sensibly  hyperbolic.  How- 
ever, out  of  at  least  a  hundred  comets  whose  elements  we 
know,  none  has  appeared  to  move  in  a  hyperbola;  it  must 
be  then  that  the  circumstances  which  produce  a  sensible 
hyperbola  are  extremely  rare  in  relation  to  the  contrary 
circumstances.  Comets  are  so  small  that  they  do  not  be- 
come visible  until  their  perihelion  distance  is  inconsider- 
able. Up  to  the  present  time  this  distance  for  any  comet 
has  only  twice  surpassed  the  diameter  of  the  terrestrial 
orbit,  and  oftenest  it  has  been  less  than  the  radius  of  that 
orbit.  It  is  obvious  that  to  approach  so  near  to  the  sun 
their  velocity  at  the  moment  of  their  entrance  into  its 
sphere  of  activity  must  have  an  amount  and  a  direction 
comprised  within  narrow  limits.  In  determining  by  anal- 


THE   SYSTEM  OF  THE   WORLD  507 

ysis  of  the  probabilities  the  ratio  of  the  chances  which,  in 
such  limits,  give  a  sensible  hyperbola  to  the  chances  which 
give  an  orbit  likely  to  be  confounded  with  a  parabola,  I 
have  found  that  there  are  at  least  six  thousand  chances  to 
one  that  a  nebula  which  penetrates  into  the  sphere  of 
activity  of  the  sun,  so  as  to  be  observed,  will  describe  either 
a  very  elongated  ellipse  or  a  hyperbola  that  from  the  size 
of  its  axis  will  be  sensibly  confounded  with  a  parabola  in 
the  portion  of  the  orbit  that  is  observed.  It  is  not  sur- 
prising, therefore,  that  so  far  hyperbolic  motions  have  not 
been  recognised. 

The  attraction  of  the  planets,  and  perhaps  also  the  re- 
sistance of  the  ethereal  media,  must  have  changed  many 
cometary  orbits  into  ellipses  whose  major  axes  are  much 
smaller  than  the  radius  of  the  sphere  of  activity  of  the  sun. 
This  change  may  also  result  from  the  meeting  of  these 
heavenly  bodies;  for  it  follows,  from  our  hypothesis  of  their 
formation,  that  there  must  have  been  a  prodigious  number 
of  them  in  the  solar  system,  it  having  been  possible  to 
observe  only  those  which  approach  near  enough  to  the 
sun.  It  is  to  be  believed  that  such  a  change  took  place 
for  the  orbit  of  the  comet  of  1759,  whose  major  axis  only 
exceeded  thirty-five  times  the  distance  from  the  sun  to 
the  earth.  A  still  greater  change  happened  to  the  orbits  of 
the  comets  of  1770  and  1805. 

If  a  certain  number  of  comets  penetrated  the  atmos- 
pheres of  the  sun  and  of  the  planets  at  the  epoch  of  their 
formation,  they  must  have  fallen  upon  those  bodies  in  spiral 
orbits,  and  by  their  fall  have  removed  the  planes  of  the 
orbits  and  of  the  equators  of  the  planets  from  the  plane  of 
the  solar  equator. 

If  in  the  zones  abandoned  by  the  atmosphere  of  the 
sun  there  have  been  molecules  too  volatile  to  unite  among 
themselves  or  with  the  planets,  they  must,  while  they  con- 
tinue to  revolve  around  the  sun,  offer  every  appearance 
of  the  zodiacal  light,  without  opposing  any  sensible  re- 
sistance to  the  various  bodies  of  the  planetary  system, 
because  of  their  extreme  rarity,  or  because  their  motion 


5o8 


LAPLACE 


is  very  nearly  the  same. as  that  of  the  planets  which  they 
meet. 

The  closer  examination  of  all  the  circumstances  of  this 
system  still  further  increases  the  probability  of  our  hypoth- 
esis. The  primitive  fluidity  of  the  planets  is  clearly  indi- 
cated by  the  flattening  of  their  figures,  in  conformity  with 
the  laws  of  the  mutual  attraction  of  their  molecules.  It  is, 
moreover,  proved  in  regard  to  the  earth,  by  the  regular 
diminution  of  gravity  from  the  equator  to  the  poles.  This 
state  of  original  fluidity  to  which  we  are  led  by  astronomical 
phenomena  ought  to  be  manifested  in  the  phenomena 
which  natural  history  presents  to  us.  But  to  find  it  there 
it  is  necessary  to  take  into  consideration  the  immense 
variety  of  combinations  formed  by  all  the  terrestrial  sub- 
stances mingled  in  a  vaporous  state,  when  the  lowering  of 
the  temperature  has  allowed  their  elements  to  unite.  We 
must  next  consider  the  prodigious  changes  which  such 
lowering  of  temperature  must  have  brought  about  succes- 
sively in  the  interior  and  at  the  surface  of.  the  earth,  in  all 
its  products,  in  the  constitution  and  pressure  of  the  atmos- 
phere, in  the  ocean  and  in  the  bodies  which  it  has  held  in 
solution.  Lastly,  the  abrupt  changes,  such  as  the  great 
volcanic  eruptions  which  must  have  disturbed  at  various 
times  the  regularity  of  the  changes,  must  be  noted.  Ge- 
ology, from  this  point  of  view,  which  links  it  with  astron- 
omy, might,  with  respect  to  many  objects,  acquire  the  pre- 
cise and  certain  knowledge  of  the  latter  science. 

One  of  the  most  singular  phenomena  of  the  solar  sys- 
tem is  the  strict  equality  which  has  been  observed  between 
the  angular  motions  of  rotation  and  of  revolution  of  each 
satellite.  It  is  a  wager  of  infinity  against  one  that  this 
is  not  the  result  of  chance.  The  theory  of  universal  gravi- 
tation causes  the  infinity  to  disappear  from  this  improba- 
bility, and  shows  us  that  for  the  phenomenon  to  exist  it 
suffices  that  at  the  beginning  the  two  motions  shall  differ 
but  slightly.  Then  the  attraction  of  the  planet  establishes 
a  perfect  equality  between  them,  but  at  the  same  time  it 
gives  rise  to  a  periodic  oscillation  in  that  axis  of  the  satel- 


THE  SYSTEM  OF  THE  WORLD  509 

lite  which  is  directed  toward  the  planet,  an  oscillation  the 
extent  of  which  depends  on  the  original  difference  of  the 
two  motions.  The  observations  of  Mayer  upon  the  libra- 
tion  of  the  moon  and  those  which  Messrs.  Bouvard  and 
Nicollet  have  just  made  upon  the  subject,  at  my  request, 
not  having  shown  this  oscillation,  the  difference  on  which 
it  depends  must  be  very  small.  This  points  with  extreme 
probability  to  a  special  cause,  which  at  first  has  confined 
that  difference  within  very  narrow  limits,  where  the  attrac- 
tion of  the  planet  has  been  able  to  establish  a  strict  equality 
between  the  mean  motions  of  rotation  and  revolution,  and 
which  afterward  ended  by  destroying  the  oscillation  which 
was  produced  by  such  an  equality.  Both  of  these  effects 
are  results  of  our  hypothesis,  for  it  is  obvious  that  the 
moon  in  a  vaporous  state  formed,  under  the  powerful  at- 
traction of  the  earth,  an  elongated  spheroid,  whose  major 
axis  must  have  been  constantly  directed  toward  that  planet, 
owing  to  the  readiness  with  which  vapours  yield  to  the 
slightest  forces  acting  upon  them.  The  terrestrial  attrac- 
tion continuing  to  act  in  the  same  manner  so  long  as 
the  moon  was  in  a  fluid  state  must  at  last,  by  constantly 
causing  the  two  motions  of  that  satellite  to  approximate, 
have  made  their  difference  fall  within  the  limits  where  their 
strict  equality  begins  to  establish  itself.  Afterward  this 
attraction  must  have,  little  by  little,  destroyed  the  oscilla- 
tion which  that  equality  produced  in  the  major  axis  of  the 
spheroid  directed  toward  the  earth.  It  is  thus  that  the 
fluids  which  cover  this  planet  have  destroyed  by  their  fric- 
tion and  by  their  resistance  the  original  oscillations  of  its 
axis  of  rotation  which  now  is  subject  only  to  the  nutation 
resulting  from  the  actions  of  the  sun  and  of  the  moon. 
It  is  easy  to  convince  one's  self  that  the  equality  of  the 
motions  of  rotation  and  of  revolution  of  the  satellites 
must  have  placed  an  obstacle  to  the  formation  of  rings 
and  of  secondary  satellites  by  the  atmospheres  of  those 
bodies.  Thus,  so  far,  observation  has  indicated  nothing 
of  the  sort. 

The  motions  of  the  first  three  satellites  of  Jupiter  pre- 


5io  LAPLACE 

sent  a  phenomenon  still  more  extraordinary  than  the  pre- 
ceding. It  consists  in  the  fact  that  the  mean  longitude 
of  the  first,  less  three  times  that  of  the  second,  plus  twice 
that  of  the  third,  is  constantly  equal  at  two  right  angles. 
There  is  infinity  against  one  to  be  wagered  that  this  equal- 
ity is  not  due  to  chance. 

But  we  have  just  seen  that,  in  order  to  produce  it,  it 
was  sufficient  that  the  mean  motions  of  these  three  bodies 
should  approximately  satisfy  the  ratio  which  makes  the 
mean  motion  of  the  first,  less  three  times  that  of  the  sec- 
ond, plus  twice  that  of  the  third,  equal  to  zero.  Their 
mutual  attraction  has  subsequently  established  this  relation 
rigorously,  and  furthermore  has  made  the  mean  longitude 
of  the  first  satellite  less  three  times  that  of  the  second  plus 
twice  that  of  the  third  equal  to  a  semi-circumference.  At 
the  same  time  a  periodic  inequality  has  arisen  which  de- 
pended upon  the  small  quantity  by  which  the  mean  motions 
originally  deviated  from  the  relation  we  have  just  stated. 
However  much  care  Delambre  has  taken  to  prove  this 
inequality  by  observations,  he  has  not  succeeded,  which 
proves  its  extreme  smallness,  and  which  consequently 
shows  with  great  probability  a  cause  which  has  made  it 
disappear.  According  to  our  hypothesis  the  satellites  of 
Jupiter,  immediately  after  their  formation,  did  not  move 
in  a  perfect  vacuum;  the  least  condensable  molecules  of 
the  original  atmospheres  of  the  sun  and  of  the  planet 
formed  then  a  rare  medium  whose  resistance,  differing  for 
each  of  the  heavenly  bodies,  caused  their  mean  motions  of 
the  relation  in  question  to  approximate  little  by  little. 
When  these  motions  have  thus  attained  the  conditions  re- 
quired in  order  that  the  mutual  attraction  of  the  three 
satellites  should  establish  this  relation  rigorously,  the  same 
resistance  constantly  diminished  the  inequality  which  the 
relation  caused  at  first,  and  finally  rendered  it  insensible. 
We  can  not  better  compare  these  effects  than  to  the  motion 
of  a  pendulum  moving  by  great  velocity  in  a  medium  of 
small  resistance.  At  first  it  will  describe  a  great  number 
of  circumferences,  but  at  last  its  motion  of  revolution,  con- 


THE   SYSTEM  OF  THE  WORLD  511 

stantly  decreasing,  will  be  changed  into  a  motion  of  oscil- 
lation, which,  itself  diminishing  more  and  more  by  the 
resistance  of  the  medium,  will  finally  be  destroyed;  then 
the  pendulum,  having  come  to  rest,  will  forever  remain 
in  that  condition. 


ARE  THE  PLANETS  HABITABLE 


BY 

GEORGE   M.   SEARLE 


33 


ARE  THE   PLANETS   HABITABLE1 

HAVING  completed  our  survey  of  the  planetary  sys- 
tem in  which  we  live,  a  question  naturally  occurs  to 
us,  which  has  occurred  to  every  inquiring  mind 
since  the  real  dimensions  of  the  orbs  belonging  to  it  were 
known.  To  the  great  majority  of  mankind  it  is,  and  is 
rightly,  a  question  of  greater  interest  than  any  one  with 
which  mathematics  or  physics  has  to  deal;  of  greater  in- 
terest, since  life  is  a  much  higher  and  nobler  thing  than 
machinery,  and  the  spiritual  far  above  the  material.  This 
question  is,  "  Are  these  planets  which,  like  our  earth,  move 
in  their  appointed  paths  around  the  sun,  and  on  which 
there  is  certainly  ample  room  for  a  population  far  greater 
than  what  our  globe  could  support,  actually  inhabited  by 
beings  in  any  way  like  ourselves?  " 

Almost  every  astronomer  has  probably  been  asked  what 
his  views  are  on  this  question,  and  whether  his  science  has 
anything  to  tell  us  about  it.  At  each  successive  increase 
in  the  size  of  telescopes,  men  vaguely  hope  that  with  the 
new  optical  power  it  may  be  possible  to  discover  some  signs 
of  sentient,  and  perhaps  even  of  intelligent,  life  in  the 
celestial  worlds.  "  How  much  does  this  telescope  mag- 
nify? "  is  always  the  interesting  question  to  the  popular 
mind.  The  professional  astronomer  perhaps  is  not  looking 
so  much  for  that.  He  wants  to  get  more  light;  to  see  and 
to  delineate  faint  nebulae,  to  follow  a  comet  as  far  as  he 
can  into  the  darkness  of  space,  in  order  to  determine  its 
orbit  as  well  as  possible;  but  the  world  in  general  has  com- 
paratively little  sympathy  with  him  in  this.  The  discovery 

1  A  lecture  delivered  before  the  Catholic  University  of  America. 
515 


516  SEARLE 

of  one  intelligent  being  outside  this  planet  of  ours  would 
be  more  interesting  to  most  men  here  than  all  the  comets 
which  ever  have  been  or  ever  will  be  seen. 

Is  it  then  possible  that  the  power  of  telescopes  will  at 
any  time  be  so  increased  that  any  discovery  of  this  kind  can 
be  made?  That  is  what  people  would  like  to  know.  Let 
us  answer  this  question  in  the  first  place. 

The  moon  is  our  nearest  neighbour.  If  we  can  magnify 
enough  to  see  an  object  the  size  of  a  man  on  any  of  the 
planetary  orbs,  we  must  first  be  able  to  see  such  an  object 
on  the  moon.  Is  it  possible  to  obtain  a  magnifying  power 
sufficient  for  this? 

It  is  possible,  we  answer,  to  have  such  a  magnifying 
power;  but  the  difficulty  is  to  avail  ourselves  of  such  a 
power  when  we  have  got  it.  The  great  and  turbulent  sea 
of  atmosphere  which  lies  above  us  is  a  seemingly  insuper- 
able difficulty.  To  some  extent,  of  course,  we  can  get  free 
from  this  by  placing  our  telescope  on  some  high  mountain; 
but  thefe  is  no  mountain  high  enough  to  place  us  alto- 
gether out  of  the  atmosphere,  and  if  there  were  one,  we 
could  not  live  or  carry  a  telescope  there.  At  the  highest 
point  at  which  observations  would  be  possible,  which  prob- 
ably would  be  a  good  deal  below  the  summit  of  the  Hima- 
layas, enough  air  still  would  remain  above  us  to  prevent 
our  using  a  power  high  enough  to  discern  men  like  our- 
selves on  the  face  of  our  satellite.  The  tremulousness  and 
waviness  produced  in  the  telescopic  image  by  the  air, 
which  is,  of  course,  increased  the  more  we  magnify,  would 
hopelessly  obscure  outlines  so  delicate  as  those  here  con- 
cerned, and  make  of  such  small  points  a  simple  invisible 
blur. 

Even  for  the  moon,  then,  the  direct  discovery  of  animal 
life  by  increased  optical  power  would  seem  to  be  a  dream 
which  will  never  be  realized.  The  difficulty,  of  course,  is 
immensely  increased  for  any  other  celestial  object.  No 
other  planet  comes  nearer  to  us  than  about  one  hundred 
times  the  moon's  distance;  and,  moreover,  in  examining 
them,  we  should  have  to  contend  with  the  confusion  of 


ARE   THE   PLANETS  HABITABLE  517 

outlines  coming  from  their  atmospheres  as  well  as  from 
our  own. 

We  may  then  as  well  give  up  hope  of  trying  to  answer 
the  question,  "  Are  the  planets  inhabited?  "  as  one  which 
never  will  be  solved  for  us  in  this  world  by  any  natural 
means;  and  fall  back  on  another,  on  which  science,  cer- 
tainly, can  give  us  some  light — namely,  "  Are  they  habit- 
able; are  the  physical  conditions  such  in  them,  so  far  as 
we  can  ascertain,  that  the  life  of  man  or  of  any  highly 
organized  animal  could  there  subsist?  " 

Now,  I  say  the  "  planets  " ;  for  it  seems  to  me  that  we 
may  as  well  put  the  great  central  body  of  our  system,  the 
sun  itself,  out  of  the  question.  I  think  it  is  pretty  clear 
that  the  surface  at  least  of  this  enormous  globe  is  in  such 
a  state  as  to  make  it  absolutely  impossible  for  us  to  con- 
ceive of  any  organized  life  existing  there.  It  is  true  that 
we  do  not  know  exactly  how  much  complexity  of  struc- 
ture is  required  in  matter  as  a  basis  of  life;  but  we  can 
hardly  consider  life  in  the  proper  sense  as  belonging  to  a 
chemical  molecule,  and  everything  would  indicate  that  on 
the  surface  of  the  sun  matter  is  reduced  to  its  simply 
chemical  or  molecular  state.  Any  structures  or  organisms 
which  we  call  alive  would  instantly  be  destroyed  in  that 
intense  flame;  even  inanimate  shapes,  like  those  of  crystals, 
would  not  survive  its  action  for  a  moment. 

But  may  there  not  be  a  cooler  region  below  the  sun's 
surface,  protected  in  some  way  from  the  intense  heat  of 
the  exterior?  Such  a  theory  was  entertained  in  the  last 
century  and  even  in  this;  but  it  is  pretty  safe  to  say  that 
no  one  now  would  hold  it.  That  it  should  have  held  its 
ground  so  long  is  due  perhaps,  in  great  measure,  to  the 
authority  of  Sir  William  Herschel.  I  do  not  think  it  was 
ever  satisfactorily  explained  just  how  the  interior  was  pro- 
tected from  the  immense  radiation  of  its  envelope;  cer- 
tainly it  is  hard  for  us  to  see  nowadays,  knowing  as  we 
do  the  radiating  power  of,  the  surface  (10,000  horse  power 
per  square  foot,  as  we  find  it  to  be)  how  such  a  blaze  as 
this  could  even  be  supposed  to  be  cut  off  from  any  point 


SEARLE 

within.  To  suggest  a  cool  place  in  the  interior  of  the  sun 
is  much  as  if  one  should  advise  a  person  suffering  from 
the  heat  of  a  furnace  to  wrap  himself  up  well  and  take  a 
seat  inside.  Moreover,  we  know  from  spectroscopic  in- 
dications now,  particularly  from  those  of  oxygen  in  the 
sun,  that  the  farther  in  we  go,  the  hotter  it  gets;  and 
this  also  would  follow  from  the  only  theory  which  can  rea- 
sonably account  for  the  formation  of  the  sun,  and  the  main- 
tenance of  its  heat. 

We  may  pretty  certainly  say,  then,  that  in  any  com- 
mon-sense way  of  using  the  word,  the  sun  is  not  habitable. 
Absolutely  speaking,  of  course,  all  space  is  habitable;  there 
is  no  conclusive  reason  why  an  organized  being  should 
require  nutriment  or  air,  and  hence  an  animal  might  be 
conceived  as  being  launched  into  space  as  a  planet  on  his 
own  account.  But  what  we  mean  by  a  place  being  habit- 
able is,  that  it  should  furnish  the  requisites  and  conven- 
iences belonging  to  a  life  similar  in  its  principal  features 
to  that  with  which  we  are  acquainted.  It  is  not  a  thing 
which  can  be  strictly  defined;  nevertheless,  we  know  well 
enough  for  practical  purposes  what  we  are  talking  about, 
and  we  know  that  such  a  place  as  this  empty  space  is  not 
"  habitable." 

From  the  consideration  of  the  sun  we  will  pass  to  that 
of  the  next  most  conspicuous  object  to  us  in  the  planetary 
system — that  is  to  say,  the  moon.  I  have  already  expressed 
in  a  previous  lecture  the  views  generally  entertained  by 
astronomers  about  the  moon.  It  is  pretty  certain  that  the 
side  of  it  which  we  see  offers  nothing  in  the  way  of  a  con- 
venience of  life  except  mere  standing-room.  There  is 
hardly  a  doubt  that  its  surface  consists  simply  of  bare  rock, 
unvaried  by  water,  soil,  or  any  kind  of  vegetation;  that  if 
there  be  any  atmosphere  upon  it,  it  is  so  excessively  rare- 
fied as  to  be,  for  purposes  of  life,  practically  equivalent 
to  none. 

As  to  the  other  side,  of  course,  we  can  say  nothing 
positively.  It  may  perhaps  in  some  way  be  different  from 
this.  But  taking  the  ordinary  and  (to  say  the  least)  very 


THE  MOON'S  SURFACE, 
Photogravure  from  a  photograph  taken  at  the  Lick  Observatory, 


SEARLE 

iin.    To  suggest  at  e  in  the  interior  of  the  sun 

men  as  if  one  sho?;  person  suffering  from 

the  heat  of  a  furnace  t-  up  well  and  take  a 

seat  inside.     Moreo  from  spectroscopic  in- 

dications now,  of  oxygen  in  the 

sun,  that  tl  !«e  hotter  it  gets;  and 

this  also  wo  which  can  rea- 

sonably i,  and  the  main- 

ny  com- 
v  habitable, 
.able;  there 
ould 


r'ut  \yhat  we  mean  by  a  place  being  h:-. 


es  belonging  to  a  life  similar  in  its  principal  features 

a  which  we  are  acquainted.     It  is  not  a  thing 

">e  strictly  defined;  nevertheless,  we  know  well 

;tical  purposes  what  we  are  talking  about, 

such  a  place  as  this  empty  space  is  not 

that 

^e  plan*  - 

entertained  by 

momers  about  the  tty  certain  that  the 

side  of  it  which  n  the  way  of  a  con- 

venience of  lifv-  i.g-room.     Ther 

hardly  a  doubt  >imply  of  bare  rock» 

unvaried  by  vegetation;  that  if 

there  be  an  it,  it  is  so  excessively  rare- 

fied as  to  be,  for  purp-  life,  pracdcally  equivalent 

to  none, 

As  to  the  other  side,  of  course,  we  can  say  nothing 

tively.    It  may  perhaps  in  some  way  be  different  from 

But  taking  the  ordinary  and  (to  say  the  least)  very 


ARE   THE   PLANETS   HABITABLE  519 

probable  view  as  to  the  method  of  formation  of  the  plan- 
etary masses,  by  cooling  from  a  liquid  condition,  it  is  hard 
to  see  how  there  could  possibly  be  any  considerable  differ- 
ence of  shape  or  of  density  between  the  half  of  the  lunar 
sphere  which  is  turned  toward  us  and  that  which  is  turned 
away.  And  unless  there  be  such  a  difference,  the  other 
side  must  be  as  destitute  of  atmosphere  as  this;  and  if  of 
atmosphere,  of  water  as  well;  for  the  water  or  other  fluid, 
if  existing  in  any  quantity,  wo.uld  form  an  atmosphere,  if 
none  previously  existed. 

The  moon  then  hardly  seems  to  present  the  condition 
required  for  what  we  should  call  a  habitable  planet;  though 
it  fails  in  a  very  different  way  from  the  sun.  The  moon  is 
dead;  the  sun  is  too  much  alive.  The  moon  may  have 
been  habitable  and  inhabited  once;  the  sun  may  be  in  the 
future. 

So  far,  our  survey  has  not  been  very  encouraging.  But 
we  have  not  yet  considered  the  planets  properly  so  called. 

In  considering  them  from  this  point  of  view,  let  us  pro- 
ceed in  the  contrary  order  to  that  which  we  followed  in 
describing  them  in  detail.  Let  us  start  at  the  outer  limit, 
with  the  great  twin  planets,  as  we  may  call  them,  on  ac- 
count of  their  great  similarity,  widely  separated  in  space 
as  they  are — namely,  Uranus  and  Neptune. 

These  would  perhaps  generally  be  imagined  as  very 
cheerless  habitations  for  intelligent  beings,  on  account  of 
their  distance  from  the  sun,  and  the  comparatively  small 
amount  of  light  and  heat  which  that  great  central  fire  sends 
to  them,  if  that  which  the  earth  receives  be  taken  as  the 
standard.  Particularly  would  this  impress  us  in  the  case 
of  Neptune.  Its  distance  from  the  sun  is  about  thirty  times 
ours,  and,  according  to  the  oft-repeated  law  of  the  inverse 
squares  of  the  distances,  the  light  and  heat  which  it  gets 
from  the  sun  is  only  one  nine-hundredth  part  of  that  which 
we  receive.  But  let  us  not  give  up  the  matter  as  hopeless 
on  this  account.  One  nine-hundredth  part  of  sunlight  is 
not  such  a  faint  illumination,  after  all.  It  is  nearly  seven 
hundred  times  the  light  of  the  full  moon,  and  indeed  equal 


5  20  SEARLE 

to  that  given  by  a  large  electric  arc-lamp  at  a  distance  of 
a  few  feet.  There  would  be  no  difficulty  about  reading  by 
means  of  it;  it  would  be  quite  sufficient  for  all  the  ordi- 
nary practical  purposes  for  which  sunlight  is  used  here. 
And  then  there  is  another  consideration  which  is  of  very 
great  weight. 

It  is  this:  You  know  that,  as  I  have  said,  what  astrono- 
mers increase  the  size  of  telescopes  for  is  to  gather  more 
light,  rather  than  to  get  greater  magnifying  power.  A 
telescope  of  two  inches  diameter,  or  aperture,  as  it  is 
technically  called,  will  give  four  times  as  much  light  as 
one  of  only  one  inch;  one  of  ten  inches  will  give  twenty- 
five  times  as  much  as  the  two-inch,  or  a  hundred  times  as 
much  as  the  one-inch.  The  great  Lick  telescope,  of  three 
feet  aperture,  makes  a  star  look  about  thirteen  hundred 
times  as  bright  as  a  one-inch  spy-glass,  and  enables  us  to 
see  stars  about  twenty  thousand  times  fainter  than  any 
which  can  be  seen  with  the  naked  eye.  And  the  same  rule 
would  hold  for  the  eye  itself.  If  we  should  increase  the 
size  of  the  pupil  of  the  eye,  we  should  see  fainter  objects 
than  we  do  now;  and  we  indeed  actually  do  this  when  we 
go  from  bright  light  into  a  dark  room.  We  can  easily  see 
how  the  pupil  dilates  to  accommodate  itself  to  reduced 
light,  by  simply  examining  another  person's  eye  in  these 
changed  conditions,  or  our  own  before  a  looking-glass. 
The  eye  of  a  cat  changes  much  more.  If  the  retina  of 
the  cat's  eye  is  as  sensitive  as  our  own,  she  must  habitually 
see  stars  five  or  six  times  fainter  than  any  which  we  can 
discern  without  a  glass,  and  the  heavens  must  present  to 
her  a  magnificent  appearance,  if  she  cares  to  look  at  them. 
Probably  she  actually  uses  this  increased  light  rather  to 
discover  mice  than  stars;  but  her  astronomical  oppor- 
tunities are  there  all  the  same,  though  she  may  not  avail 
herself  of  them. 

It  is  true  that  this  increased  light  is  obtained  in  the  eye 
at  some  sacrifice  of  definition,  or  sharpness  of  vision  in 
detail;  but  still  an  inhabitant  of  Neptune  might  have  a 
good  deal  larger  pupil  than  ours  in  proportion  to  the  size 


ARE   THE   PLANETS   HABITABLE 


521 


of  his  eye  than  ours.  And  then,  again,  there  is  no  reason 
why  the  retina  itself  should  not  be  made  much  more  sen- 
sitive to  light  than  ours;  and  here  we  have  an  increase 
which  has  no  limit,  so  far  as  we  can  tell.  It  would  be  an 
injury  to  us  to  have  our  optic  nerve  more  sensitive;  the 
strong  sunlight  to  which  we  are  exposed  would  hurt  us. 
But  there  is  no  reason  why  the  Neptunians  should  not 
have  what  .would  be  a  benefit  to  them. 

The  whole  question,  then,  of  light  in  the  solar  system 
is  one  of  little  consequence;  eyes  could  easily  in  any  planet 
be  such  as  to  suit  the  exigencies  of  the  case. 

With  regard  to  heat,  the  question  is  a  little  more  dif- 
ficult, but  not  very  much.  If  we  should  assume  that  the 
500°  Fahrenheit  by  which  our  temperature  here  is  raised 
above  that  of  space  are  simply  due  to  our  distance  from 
the  sun,  and  that  Neptune  could  only  have  one  nine-hun- 
dredth part  of  that,  of  course  the  temperature  there  would 
practically  be  that  of  space  itself,  or  460°  below  the  Fahren- 
heit zero.  But  we  know  that,  in  fact,  the  genial  warmth  of 
the  earth  is  in  a  great  measure  due  to  its  atmospheric  gar- 
ment or  blanket;  and  we  can  not  be  at  all  sure  that  an 
atmosphere  may  not  exist  on  Neptune  which  may  make 
the  absorption  so  much  greater  than  the  radiation  that  an 
equality  between  the  two  would  not  be  reached  before  the 
planet  had  accumulated  from  its  scanty  solar  supply  enough 
to  make  its  temperature  equal  to  ours. 

And,  besides,  there  is  no  certainty  that  these  great  outer 
planets  may  not  still  retain  a  great  deal  of  their  own  in- 
trinsic heat;  that  they  may  yet  be  warm  enough,  even  on 
the  surface,  to  act  as  a  source  of  heat  to  their  inhabitants. 
Indeed,  the  danger  here  is  rather  that  they  are  too  hot 
than  too  cold.  Yes,  that  is  the  trouble  with  all  the  great 
outer  planets,  with  Jupiter  and  Saturn,  as  well  as  Uranus 
and  Neptune,  as  we  shall  shortly  see.  As  far  as  atmosphere 
is  concerned,  the  spectroscope  would  indicate  rather  a  dense 
one  on  both  Uranus  and  Neptune,  and  of  the  same  char- 
acter on  each.  Uranus  shows  belts  on  its  surface  similar 
to  those  seen  on  Jupiter;  but  we  can  not  be  sure  that  this 


522  SEARLE 

indicates  a  similar  constitution  in  the  two  planets.  On  the 
whole,  we  may  say  that  there  is  quite  what  we  may  call 
a  probability  that  Uranus  and  Neptune  are  in  a  habitable 
condition;  the  probability  is,  however,  as  we  may  say, 
rather  negative  than  positive;  we  can  not  give  any  cer- 
tain reason  why  they  should  not  be;  but  there  are  really 
no  positive  indications  to  show  that  they  are  fit  to  be  the 
abode  of  life.  The  arguments  against  habitability  become 
much  stronger  in  the  case  of  the  two  giants  of  the  plan- 
etary system,  Saturn  and  Jupiter,  which  come  next  in  order 
as  we  proceed  toward  the  sun.  The  brilliancy  of  Jupiter's 
surface,  and  the  rapidity  of  the  changes  which  we  see  there, 
exceeding  what  the  moderate  light  and  heat  which  it  re- 
ceives from  the  sun  would  be  likely  to  produce,  seem  to  be 
quite  strong  arguments  that  it  is  still  in  a  condition  to 
emit  light  and  heat  to  a  considerable  extent  on  its  own 
account;  and,  indeed,  that  its  temperature  is  still  suffi- 
cient to  keep  it  in  a  fluid  state.  If  its  surface  be  indeed 
in  the  condition  of  molten  metal,  it  certainly  becomes  unin- 
habitable in  the  common-sense  view  of  the  subject;  for  in 
melted  metal  no  organism  composed  of  ordinary  chemical 
elements  could  possibly  subsist. 

These  arguments  apply  with  somewhat  diminished  force 
to  Saturn.  Another,  however,  which  may  perhaps  be  de- 
rived from  the  lightness  or  small  density  of  all  the  four 
great  exterior  planets  of  which  we  have  been  speaking,  is 
strongest  in  the  case  of  this  one.  This  lightness  may  indi- 
cate that  they  have  not  yet  shrunk  to  their  proper  dimen- 
sions; for  it  seems  reasonable  enough  to  suppose  that  the 
chemical  constituents  throughout  the  solar  system  are  the 
same;  that  all  the  planets  are  chips  out  of  the  same  block; 
and  that  when  all  are  reduced  to  the  physical  state  of  the 
earth  they  would  have  about  the  same  density.  But  this 
does  not  seem  to  amount  to  much;  for  though  it  holds  well 
enough  in  the  cases  of  Mars  and  Venus,  it  notably  fails 
in  that  of  Mercury,  if  the  determinations  of  the  mass  of 
that  planet  can  be  considered  as  trustworthy.  The  density 
of  Mercury  would  appear,  it  will  be  remembered,  to  be 


ARE   THE   PLANETS   HABITABLE  523 

twice  that  of  the  earth;  which  would  prove  most  undoubt- 
edly that  it  was  made  of  decidedly  heavier  materials,  unless 
we  maintain  that  it  is  very  much  more  solidified  than  the 
earth,  which  would  seem  to  be  improbable.  When  a  planet 
has  once  become,  like  the  earth,  solid  on  the  surface,  no 
further  perceptible  shrinkage  is  possible  except  by  a  com- 
plete breaking  up  of  the  crust,  which  could  hardly  result 
except  from  a  collision. 

But  to  return  to  the  great  planets  of  which  we  have 
been  speaking.  I  think  few,  if  any,  astronomers  believe 
them  to  be  habitable  in  their  present  condition;  for,  though 
the  case  is  more  doubtful  for  Uranus  and  Neptune,  still 
they  have,  in  their  general  features,  so  much  resemblance 
to  Jupiter  and  Saturn,  that  it  is  usually  presumed  that  they 
are  in  the  same  state.  But  no  one  could  pretend  to  be  cer- 
tain with  regard  to  the  matter. 

Before  we  leave  this  portion  of  our  system,  however,  we 
must  not  omit  a  part  of  it  which  is  eminently  worth  con- 
sidering with  reference  to  the  present  question.  I  mean  the 
numerous  satellites,  which  are  such  a  striking  feature  in  it. 

Let  us  consider  specially  those  of  Jupiter,  about  which 
we  know  the  most.  The  four  moons  of  Jupiter  are  all  quite 
considerable  bodies,  ranging  in  size  from  that  of  our  moon 
to  that  of  the  planet  Mars.  There  is  plenty  of  room  on 
them  for  a  very  large  population;  the  surface  of  the  largest 
does  not  fall  far  short  of  that  of  the  land  part  of  our  own 
globe.  There  is  no  reason  why  they  should  not  be  in  the 
same  general  physical  state  as  the  earth  is ;  we  have  already 
seen  that,  as  far  as  light  and  heat  are  concerned,  they  may 
be  considered  as  amply  provided;  perhaps,  indeed,  even 
better  than  we;  for  the  great  planet  itself,  round  which  they 
circulate,  would  probably  serve  as  a  much  better  luminary 
by  night  than  our  own  moon,  and  may  very  probably  con- 
tribute not  a  little  to  keeping  them  comfortably  warm,  if 
it  is  indeed  still  in  a  melted  and  glowing  condition.  We 
may  well  believe  that  it  is  indeed  a  second  sun  to  them, 
and  if  the  satellites  of  Jupiter  keep,  like  our  own  moon,  the 
same  side  always  turned  toward  the  primary  planet,  that 


524 


SEARLE 


favoured  side  would  enjoy  a  continual  warmth,  which  might 
indeed  be  excessive. 

Similar  remarks  may,  of  course,  be  made  of  all  the  other 
satellites  which  we  find  in  this  great  region,  revolving 
round  Saturn,  Uranus,  and  Neptune.  Much  has  been  said 
of  the  splendour  of  the  Saturnian  sky  as  seen  from  the 
planet  itself,  with  the  great  ring  arching  over  the  heavens 
and  the  satellites  circling  along  it.  It  is  far  more  likely 
that,  if  this  splendour  is  seen  at  all,  it  is  from  the  satellites, 
from  which,  especially  from  Japetus,  the  most  remote, 
whose  orbit  lies  outside  of  the  plane  of  the  ring,  a  most 
magnificent  view  of  the  noble  planet,  with  its  rings  and 
the  other  satellites,  could  be  had.  Saturn  from  Japetus 
would  look  as  it  does  to  us  with  a  magnifying  power  of 
about  three  hundred  and  fifty  diameters;  or,  to  use  another 
illustration,  the  ball  of  the  planet  would  look  about  three 
and  a  half  times  the  diameter  of  the  moon,  and  the  rings 
nearly  nine  times  that  diameter. 

We  come  next,  in  our  inward  course,  to  the  planet 
Mars.  Here,  for  the  first  time,  we  begin  to  see  positive 
signs,  instead  of  mere  negative  possibilities,  of  what  we 
have  been  looking  for. 

We  have  noticed,  as  we  passed  this  planet  on  our  way 
outward  from  the  sun,  the  similarity  of  its  surface  to  that 
of  the  earth,  the  permanent  configurations  on  it  of  what 
we  have  a  good  right  to  assume  to  be  land  and  water.  We 
have  seen  its  polar  ice-caps,  its  green  seas,  and  red  earth; 
and  we  know  that  it  has  an  atmosphere  which,  though  not 
as  dense  as  our  own,  is  still  enough,  as  it  would  seem,  for 
life.  We  know  that  it  has  a  day  almost  exactly  the  same 
as  ours,  and  not  only  this,  but  seasons  substantially  like  our 
own,  as  far  as  the  varying  angle  is  concerned  at  which  the 
sun's  rays  strike  its  surface,  though  it  is  true  that  these  are 
a  good  deal  interfered  with  by  the  considerable  variation 
in  the  sun's  heat,  depending  on  the  eccentricity  of  its  orbit; 
still  this  would  not  amount  to  so  very  much.  In  this  lati- 
tude, for  instance,  on  the  earth,  we  receive  more  than  three 
times  the  heat  from  the  sun  in  one  day  in  the  middle  of 


ARE   THE   PLANETS   HABITABLE  525 

June  than  we  get  in  the  middle  of  December,  on  any  given 
area,  say  a  square  mile  or  a  square  yard,  owing  to  the  com- 
bined influence  of  the  greater  height  of  the  sun  above  the 
horizon  and  the  greater  length  of  the  daylight.  About  the 
same  would  be  the  case  in  the  same  latitude  on  Mars.  The 
effect  of  the  eccentricity  would  be  quite  considerable,  mak- 
ing the  sun's  heat  once  and  a  half  as  great  at  the  nearest 
point  as  at  the  farthest;  still,  if  we  can  sustain  the  three- 
fold multiplication,  a  half  as  much  again  might  be  added, 
without  the  variation  becoming  intolerable.  Moreover,  this 
great  variation  would  only  occur  when  the  summer  solstice 
of  one  of  the  hemispheres  coincided  with  the  point  of  near- 
est approach  to  the  sun.  During  half  the  time,  the  eccen- 
tricity would  tend  to  moderate,  instead  of  to  accentuate,  the 
seasons,  as  it  does  with  us  here  in  the  northern  hemi- 
sphere now. 

Mars  is  certainly  the  most  favourable  case  for  those  who 
would  believe  the  planets  to  be  habitable.  It  really  seems 
that  it  might  be  inhabited  by  men  like  ourselves.  As  re- 
marked  on  a  previous  occasion,  its  climate  seems,  from  the 
small  size  of  the  polar  ice-caps,  to  be  warmer  than  that  of 
the  earth,  in  spite  of  its  greater  distance  from  the  sun. 

As  to  Venus  and  Mercury,  we  can  hardly  form  any  de- 
cided opinion.  They  seem  to  be  surrounded  by  dense, 
cloudy  atmospheres,  which  may  tend,  in  a  great  measure, 
to  keep  off  the  intense  heat  of  the  sun.  A  rather  singular 
thing  has  lately  been  observed,  or  at  least  thought  to  be 
observed,  by  Schiaparelli,  with  regard  to  Mercury — that  is, 
that  some  markings  on  it  seem  to  indicate  that  its  period 
of  rotation  round  its  axis  is  the  same  as  that  of  its  revolu- 
tion round  the  sun;  or,  in  other  words,  that  it  acts  as  our 
moon  does,  keeping  always  the  same  face  toward  the  centre 
round  which  it  revolves.  This  would  seem  to  be  borne  out 
by  the  white  spot  on  the  black  disk  of  the  planet,  which 
has  been  reported  by  various  observers  as  regularly  visible 
at  the  time  of  its  transits  across  the  sun's  face.  If  this 
white  spot  is  a  real  object,  it  would  seem  that  it  is  always 
turned  away  from  the  sun.  If  this  can  be  accepted,  it  would 


526 


SEARLE 


be,  of  course,  to  some  extent,  an  argument  against  the 
habitability  of  Mercury,  as  its  inhabitants  would  be  de- 
prived of  the  vicissitude  of  day  and  night,  and  the  side 
turned  constantly  toward  the  sun  would  probably,  in  spite 
of  everything,  become  uncomfortably  warm. 

Now  that  we  have — though  quite  hurriedly — completed 
our  consideration  of  the  planets  as  to  their  suitableness  for 
habitation,  what  answer  shall  we  give  to  the  question  with 
which  we  started?  Before  giving  it,  another  reflection  must 
be  made,  which  will  brighten  the  prospect  a  good  deal  for 
those  who  would  fain  believe  all  these  magnificent  orbs  to 
be  the  abode  of  life  like  ours. 

It  is  this:  Will  it  not  suffice  to  satisfy  the  minds  of  those 
who  can  not  believe  that  these  great  globes,  similar  in  so 
many  respects  to  ours,  can  be  tenantless,  to  hold  that  they 
are  habited  for  a  portion,  though  not  for  the  whole,  of  their 
history?  For  myself,  I  do  not  feel  the  craving  for  the 
plurality  of  worlds,  as  it  is  called,  which  seems  to  be  gen- 
eral. I  must  confess  that  I  have  never  been  able,  per- 
sonally, to  feel  the  force  of  the  argument  which  strikes  most 
minds  so  powerfully,  that  these  habitations  could  not  have 
been  made  by  their  Creator  except  to  be  actually  inhabited. 
The  mere  size  and  mass  of  an  object  seem  to  me  to  amount 
to  little.  Jupiter  itself,  or  Saturn,  with  its  beautiful  ring 
and  satellite  system,  simply  as  a  mass  of  matter  or  a  me- 
chanical construction,  is  a  far  less  noble  creation  of  God 
than  a  single  human  soul;  nor  does  it  seem  to  me  that 
the  mere  size  of  these  planets  makes  them  much  more 
remarkable,  or  requires  more  reason  for  their  formation, 
than  if  they  were  only  a  few  feet  in  diameter.  The  technical 
study  of  astronomy,  no  doubt,  has  the  effect  of  reducing 
the  impression  made  by  mere  magnitude  on  the  mind; 
whether  this  is  a  delusion  or  the  removal  of  a  delusion,  of 
course  I  can  not  say.  That  the  mere  size  of  a  body  itself 
does  not  require  inhabitants  for  it,  seems  plain  from  the 
generally  confessed  impossibility  of  inhabiting  the  sun,  the 
surface  of  which  far  exceeds  that  of  all  the  planets  put  to- 
gether— that  is  to  say,  that  it  does  not  require  them  at 


ARE   THE   PLANETS   HABITABLE  527 

every  moment;  but  it  may  be,  if  you  will,  that  it  does  re- 
quire that  at  some  time  or  other  it  should  be  used  for  such 
a  purpose.  The  general  belief  is,  we  may  say,  an  argu- 
ment for  the  fact. 

And,  of  course,  the  argument  for  the  plurality  of  worlds 
is  strengthened  if,  besides  size  or  standing-room,  as  we  may 
say,  we  see  some  other  conditions  indicating  conveniences 
for  life,  though  they  be  imperfect  or  incomplete.  If  we  see 
a  house  with  only  its  framework  up,  we  say,  "  Nobody  lives 
there  now,  but  it  is  being  built  for  some  one  ";  and  if  we 
see  a  house  in  ruins,  we  say,  "  Somebody  lived  there  once." 

Now,  this  is  certainly  very  plausible;  and  I  think  that 
the  history  of  our  own  earth,  so  far  as  it  can  be  learned 
from  science,  increases  the  probability  of  the  opinion  that 
the  planets,  and  perhaps  even  the  sun  itself,  were  made  to 
be  inhabited  at  some  time  or  other.  The  teaching  of 
geology  is  that  our  own  earth  was  for  a  long  time  unin- 
habitable; that  it  subsequently  became  fitted  to  be  the 
abode  of  the  inferior  and  simpler  forms  of  life,  and  finally 
became  ready  for  the  reception  of  man ;  and  we  can  hardly 
shut  our  eyes,  either,  to  the  scientific  conclusion  that,  from 
the  operation  of  natural  causes  alone,  it  would  at  some  time 
in  the  distant  future  become  uninhabitable  again,  though 
in  a  different  way;  that  it  would  become,  simply  from  the 
changes  which  must  come  from  the  gradual  progress  of 
cooling  necessarily  going  on  in  the  solar  system,  no  longer 
a  building  which  its  Creator  is  forming,  but  a  cold  and 
desolate  ruin  like  the  moon. 

The  history  of  this  earth  is  probably  the  history  of  the 
other  planets,  if  they  are  to  be  allowed  to  develop  in  a 
natural  way.  Some,  like  the  moon,  seem  to  have  passed 
farther  along  the  road  than  our  own  planet.  This  is 
probably  the  case  with  Mars,  the  most  habitable  in  appear- 
ance of  them  all.  As  a  rule,  of  course,  the  smaller  a  planet 
is,  other  things  being  equal,  the  more  rapidly  it  will  cool 
from  its  originally  incandescent  state;  Mars  then  should 
be  older — that  is,  have  passed  through  more  of  its  succes- 
sive changes — than  we.  It  looks  so,  besides.  The  seas 


528  SEARLE 

seem  to  be  drying  up,  the  air  thinning  away.  On  the  other 
hand,  the  great  superior  planets,  Jupiter,  Saturn,  Uranus, 
and  Neptune  are  young,  and  have  the  best  part  of  their 
life  before  them. 

What  portion  of  the  total  life  of  a  planet  is  that  in 
which  it  becomes  habitable  by  beings  like  ourselves  we 
can  not  very  well  determine.  If  we  accept  the  estimates 
of  geology,  the  time  that  the  human  race  has  been  here  is 
a  very  small  part  of  our  world's  history.  But  how  much 
longer  this  earth  would  naturally  remain  a  possible  resi- 
dence for  us  we  can  not  say  with  accuracy.  It  would  seem 
probable,  however,  that  the  period  in  which  all  the  neces- 
sary conditions  of  life  would  simultaneously  exist  can 
hardly  be  a  very  considerable  part  of  the  whole.  The  in- 
habitants of  a  planet  in  the  stage  of  decadence  from  its 
most  perfect  state  could,  no  doubt,  on  the  principle  of  the 
"  survival  of  the  fittest,"  accommodate  themselves  to  their 
more  unfavourable  circumstances  for  a  good  while;  but 
the  time  would  come  when  the  struggle  would  have  to  be 
abandoned. 

If  it  is  true  that  the  period  of  habitability  by  the  high 
organisms  is  a  small  part  of  a  planet's  life,  obviously  the 
chance  is  small  for  any  planet  in  particular  of  its  being  in 
that  period  now  or  at  any  particular  time.  We  must  say 
that  it  probably  is  not,  unless  we  have,  as  in  the  case  of 
Mars,  some  positive  indications  that  it  is.  So  far  as  we 
can  trust  such  positive  indications,  Venus  and  Mercury  are 
approaching  that  part  of  their  life  that  the  earth  is  in  at 
present;  the  earth  seems  at  one  time  to  have  had  the  very 
dense  and  vaporous  atmosphere  that  apparently  surrounds 
them  now. 

To  sum  up  now,  briefly,  the  results  to  which  our  exam- 
ination has  led  us:  In  the  first  place,  our  observations 
should  probably  be  modified  by  the  very  plausible  theory, 
now  generally  adopted,  that  all  the  bodies  of  our  system, 
sun  and  planets,  have  passed  and  are  passing  through  a 
series  of  changes,  beginning  with  a  state  of  great  heat  and 
expansion,  in  which  and  for  a  long  time  no  life  is  possible 


ARE   THE   PLANETS   HABITABLE 


529 


on  their  surfaces,  and  in  a  great  part  of  which  indeed,  as 
in  the  case  of  the  sun  at  present,  they  can  hardly  be  said 
to  have  a  surface  at  all.  As  the  changes  due  to  the  gradual 
cooling  and  contraction  proceed,  life  in  its  simpler  forms 
becomes  possible,  and  in  course  of  time  a  state  is  reached 
like  that  of  this  globe  at  present,  in  which  the  conditions 
for  highly  organized  life  are  at  their  best. 

Assuming  this,  the  question  of  fact  becomes,  Is  there 
any  other  planet  or  satellite  in  the  system  in  which  this 
state  of  maximum  habitability,  if  we  may  so  call  it,  now 
exists?  We  can  say  with  great  confidence  that  it  does  not 
on  Jupiter  and  Saturn;  that  the  chances  are  much  against 
it  on  Uranus  and  Neptune;  that  Venus  and  Mercury  are 
probably  still  too  young  for  it;  but  that  there  is  a  reason- 
able probability  for  it  on  Mars,  though  this  planet  seems 
to  be  passing  into  the  decline,  the  steps  of  which  we  do 
not  clearly  understand,  but  of  which  we  see  perhaps  the 
final  result  in  the  torn,  scarred,  and  desolate  surface  of  our 
own  satellite.  With  regard  to  the  satellites  of  the  great 
planets,  we  have  absolutely  to  suspend  judgment.  As  the 
period  of  habitability  is  probably  less  than  that  of  develop- 
ment, though  of  this  we  are  far  from  certain,  the  chances 
are  perhaps  against  any  particular  one  of  them  being  in 
that  state  just  now;  but  as  they  number  at  least  seventeen 
altogether,  the  probability  that  some  one  of  them  may  be 
habitable  is  not  so  inconsiderable.  As  to  the  satellites  of 
Mars,  and  the  swarm  of  asteroids,  they  seem  to  be  too 
small  to  retain  an  atmosphere  sufficient  for  the  support  of 
beings  like  ourselves.  If  they  had  a  course  to  run,  it  has 
probably  been  concluded  long  ago. 

In  speaking  of  the  natural  life  and  development  of  the 
planets,  we  are,  of  course,  looking  at  the  matter  merely 
from  a  scientific  point  of  view.  Of  course,  most  Christians 
believe  that  long  before  the  natural  life  of  this  earth  would 
be  concluded,  it  will  suffer  a  final  catastrophe  which  will 
at  least  close  the  history  of  the  human  race  on  it  as  it 
exists  now.  Such  catastrophes  may,  of  course,  occur  to 
any  planet  by  natural  as  well  as  supernatural  causes;  by 

34 


530 


SEARLE 


collision  with  some  other  body,  for  instance;  or  to  the 
whole  planetary  system,  by  some  large  body  striking  on 
the  sun.  One  thing  which  we  may  perhaps  look  forward 
to  is  a  time  when,  after  the  death  or  destruction  of  all  the 
planets,  the  sun  itself  ceasing  to  be  a  luminary  and  furnace 
for  bodies  circulating  round  it,  may  itself  become  the  great 
seat  and  home  of  life.  In  theorizing  on  this  point  we 
have  no  past  experience  or  history  to  guide  us.  We  shall 
see  as  we  go  on  to  discuss  the  stellar  systems  that  we  have 
at  least  one  case,  perhaps  more  than  one,  of  a  body  sunlike 
in  dimensions,  which  has  either  ceased  to  give  light  or 
never  gave  it.  It  is  only  in  exceptional  cases  that  we  have 
any  means  of  recognising  the  existence  of  such  bodies; 
they  may  be  very  numerous.  Neither  can  we  tell  whether 
the  other  innumerable  brilliant  suns  scattered  through 
space  have  attendant  planets  like  our  own.  But  it  would 
be  strange  if  they  had  not.  If  any  considerable  proportion 
of  them  have,  evidently  the  chance  that  there  are  other 
habitable  worlds  in  the  universe  becomes  very  great. 


INDEX 


Aberration,   203;    chromatic,   347, 

350;  spherical,  347,  350. 
Acidalium,    Mare,    Martian    sea, 

145,  151,  152. 

Ackeron,  Canal  of  Mars,  156. 
^Ethra,  the  asteroid,  243. 
Airy,  geodetic  values  of,  108. 
Aldebaran  (o  Tauri),  259;  proper 

motion    of,    279;    spectrum    of, 

373- 
Algol  (0    Persei),  462;   spectrum 

of,  429. 

"Almagest"  of  Ptolemy,  256,  258. 
Andromeda,  nebula  in,    169,  417, 

437,  494;  spectrum  of,  386. 
Aonius,    Sinus,    Martian    estuary, 

I54-. 

Aquarius,  nebula  in,  384. 

Arcturus  (a  Bootis),  proper  mo- 
tion of,  272,  279,  426. 

Argelander,  "Durchmusterungen" 
of,  259,  264. 

Argyre,  Martian  island,  154. 

Aristarchus  of  Samos,  method  of 
determining  solar  parallax,  188. 

Asteroids,  216;  discovery  of,  303; 
celestial  photography  first  used 
in  discovery  of,  242;  origin  of, 
Olbers's  theory,  249;  value  in 
determining  parallax,  197,  244. 

Astronomical  publications,  313. 

Astronomy,  American,  309;  origin 
of,  255;  sidereal,  255. 

Astrophysics,  291,  363,  441. 

Atlantis,  Martian  peninsula,  150. 

"  Atlas  Ccelestis,"  Flamsteed's, 
258. 

Aurigae,  a,  see  Capella. 

Aurigae,    3,  spectrum  of,  428,  489. 

Aurigae,  C    spectrum  of,  431. 

Aurora  borealis,  spectrum  of  the, 
401. 

Aurorae,  Sinus,  Martian  estuary, 
154- 

Ausonia,  Martian  peninsula,  150. 

Australe,  Mare,  Martian  sea,  149, 

iSi,  154,  155- 
Auzout,  great  telescope  of,  344. 


Baltia,  a  district  of  Mars,  151. 
Barnard,     celestial     photographs 

taken  by,  22. 
Berlin  Academy,  star  maps  of  the, 

262. 
Bessel,    geodetic   values    of,    108; 

parallax  of  61  Cygni,  323. 
Betelgeux   (a    Orionis),   spectrum 

of,  373,  379- 

Bond,  William  Cranch,  318. 
Bootis,  o,  see  Arcturus. 
Bowditch,  Nathaniel,  314. 
Bradley,    observations    of    stellar 

parallax,  325. 

Brashear,    methods   of  lens-mak- 
ing, 357- 
Brewster,  Sir  David,  204;  theory 

of  life  in  other  worlds,  83. 
Brucia,  the  asteroid,  243. 
Buffon,  system  of  the  world,  499. 

Campani,     improvement     of    the 

telescope,  344. 
Canals  of  Mars,  153;  gemination 

of,  156. 
Capella  (a  Aurigae),  256;  spectrum 

of,  395,  469,  486. 
Capricorni,    ft,  spectrum  of,  431. 
Cassini,  discoveries  of,  345;  early 

determinations  of  parallax,  189. 
Castor    (a    Geminorum),    proper 

motion  of,  279. 
Centauri,  o,  parallax  of,  331. 
Cephei,  /i»  spectrum  of,  379. 
Ceres,  the  asteroid,  discovery  of, 

241. 

Ceti,  31;  spectrum  of,  431. 
Chandler,    investigation   of   polar 

movements,  32. 
Chauvenet,  312. 
Cimmerium,  Martian  sea,  150. 
Clark,  Alvan,  353. 
Clark,  geodetic  values  of,  108. 
Clusters,  star,  385. 
Coal,  decrease  in  supply  of,  76. 
Coast  Survey,  United  States,  313. 
Comets,  22;   Laplace's  theory  of, 

506;   connection   with   meteoric 


532 


INDEX 


swarms,  182,  402,  463;  photo- 
graphs of,  299;  spectra  of,  387, 
402,  463,  470. 

Constellations,  the,  258. 

Copernicus,  system  of  astronomy, 
24,  187. 

Corona,  of  the  sun,  405,  465. 

Crumpling,  investigations  of 
earth-,  122. 

Cygni,  0,  spectrum  of,  378. 

Cygni,  61,  parallax  of,  280,  323, 
327. 

Cygnus,  constellation  of,  166. 

Darwin,  George  Howard,  mete- 
oric nebular  theory,  126,  163, 
417;  tidal  theory  of  satellite  for- 
mation, 250. 

Deucalionis,  Regio,  peninsula  of 
Mars,  151,  154. 

Divini,  improvement  of  the  tele- 
scope, 344. 

Dog  Star,  see  Sirius. 

Dolland,  John,  improvement  of 
the  telescope,  351. 

Draconis,  a,  28. 

"  Durchmusterungen,"  264. 

Earth,  the,  216;  astronomical  ap- 
pearance of,  148,  1 60;  density  of. 
method  of  determining,  199; 
evolution  of,  64;  effects  of  cool- 
ing of,  71,  115,  120,  122;  form 
of,  controversy  of  the  Newto- 
nians and  Cassinians,  106;  mass 
of,  distribution  of  the,  in; 
mathematical  theories  of,  103; 
orbit  of,  222;  past  and  future  of, 
55;  physical  condition  of,  39; 
rotation  of,  30;  shape  of,  198; 
size  of,  methods  of  determining, 
107. 

Earthquakes,  124;  cause  of,  77. 

Eclipses,  photographs  of,  295. 

Elysium,  district  of  Mars,  149. 

Encke's  value  of  solar  parallax, 
209. 

Equation,  astronomical  meaning 
of,  203;  personal,  478. 

Equilibrium,  kinds  of,  235. 

Eros,  the  asteroid,  discovery  of, 
241,  243;  value  of,  in  determin- 
ing parallax,  245. 

Erythraeum,  Mare,  Martian  ocean, 

144,  151,  154,  155- 
Eskimos,  astronomy  of  the,  256. 
Ether,  luminiferous,  204. 


Eudoxus,  celestial  globe  of,  256. 
Euler,  theories   of,   in   regard  to 
aberration,  350. 

Faraday,   magnetization   of  light, 

204. 
Fizeau,   measurement  of  the   ve- 

locity of  light,  192,  201. 
Flamsteed,  "  Atlas  Ccelestis,"  258; 

determinations  of  parallax,  189. 
Fons  Juventae,  Martian  lake,  153. 
Forests,  devastation  of,  76. 
Foucault,  method  of  determining 

velocity  of  light,  202. 
Fournier,  theory  of  heat  diffusion, 

US- 

Franklin,   Benjamin,   309. 
Fraunhofer,   improvement   of  the 

telescope,  352;  discovery  of  the 

lines  of  the  solar  spectrum,  445. 

Galileo,   astronomical   discoveries 

of,  342;  telescope,  claim  to  the 

invention  of  the,  189,  342. 
Gas,  meteorites  acting  as  a  (Dar- 

win's   theory),     126,     163,    417; 

molecular  theory  of,  10,  15. 
Gascoigne,   William,   inventor   of 

the  micrometer,  189. 
Geminorum,  o,  see  Castor;  j8,  see 

Pollux. 

Geodesy,  development  of,  107. 
Geological    time,    Lord    Kelvin's 

limits  to,  121. 
Gilliss,  James  M.,  317. 
Glass,    optical,    manufacture     of, 

351- 
Godfrey,  inventor  of  the  sextant, 

310. 
Gould,  "  Uranometria  Argentina," 

259- 
Gravitation,  force  of,  at  surface  of 

the  sun,  17;  law  of,  218;  use  of, 

in    determining    parallax,    192; 

variation  of,  with  latitude,   no. 
Guinaud,     improvements    in    the 

manufacture    of    optical    glass, 


Hadley,  reflector  of,  348. 
Hadriacum,   Martian  ocean,  150. 
Halley,  observation  of  transits  of 

Mercury  and  Venus,  190. 
Heat,   effect  of,   upon  molecules, 

9,  13,  15;  internal,  of  earth,  70, 

115;  specific,  46  note;  sun's,  see 

Sun. 


INDEX 


533 


Helmholtz,  theory  of  the  sun's 
heat,  3,  51. 

Henderson,  observations  of  stellar 
parallax,  326,  331. 

Henry,  Joseph,  316. 

Hercules,  star  cluster  in,  386. 

Herculis,  a,  spectrum  of,  377. 

Herschel,  Sir  William,  discover- 
ies of,  241,  348. 

Hesperia,  Martian  peninsula,  150. 

Hilda,  the  asteroid,  243. 

Hipparchus,  astronomical  discov- 
eries of,  25,  256;  method  of  de- 
termining solar  parallax,  189. 

Holmes's  comet,  22. 

Huygens,  discovery  of  the  rings 
of  Saturn,  345;  reflector  of, 
348. 

Hydaspes,  canal  of  Mars,  156. 

Hydrogen,  spectrum  of,  16. 

Hyperboreus,  Lacus,  Martian 
lake,  145,  152. 

Ingeborg,  the  asteroid,  243. 

Iris,  the  asteroid,  246. 

Ismenius,    Lacus,    Martian    lake, 

152,  154. 
Isostasy,  theory  of,  123. 

Jupiter,  the  planet,  habitability  of, 
522;  mass  of,  61;  size  and  dis- 
tance, 216;  spectrum  of,  372. 

Jupiter,  satellites  of,  84,  232,  233; 
discovered  by  Galileo,  342;  fifth 
satellite  discovered  by  Barnard, 
22;  habitability  of,  523;  rota- 
tion of,  509;  use  in  determining 
light  equation,  203. 

Kant,  nebular  hypothesis  of,  167, 
170. 

Kelvin,  Lord  (Sir  William  Thom- 
son), terrestrial  temperatures, 
119;  molecular  theory,  14. 

Kirchhoff,  interpretation  of  the 
spectrum,  445;  development  of 
the  spectroscope,  366. 

Kyrae,  0,  spectrum  of,  431. 

Laplace,  distribution  of  earth's 
mass,  in;  see  Nebular  Hypoth- 
esis. 

Latitude,  method  of  determining, 
34;  variations  in,  due  to  polar 
changes,  36;  value  of,  in  astro- 
nomical investigation,  38. 

Leibnitz,  "  consistentior  status  " 
of,  115,  117. 


Lens-making,  354. 

Leonis,  a,  see  Regulus. 

Leverrier,  observations  of  Mer- 
cury, 133. 

Libration,  135;  of  Mercury,  136; 
of  the  moon,  135. 

Lick  Observatory,  306. 

Life,  sources  of,  70  note,  72. 

Light,  composite  character  of, 
443;  corpuscular  theory  of,  203; 
magnetization  of,  204;  velocity 
of,  measurement  of,  192,  201, 
use  in  determining  parallax, 
192,  201. 

Lilienthal,  observations  of  Mer- 
cury, 133. 

Lippersheim,  see  Lippershey. 

Lippershey,  Franz,  inventor  of 
the  telescope,  344. 

Liquids,  molecular  theory  of,  10, 
12. 

Lunae,  Lacus,  Martian  lake,   152, 

154- 

Lunar  inequality,  197. 
Lyra,  nebula  in,  385,  388. 
Lyne,  «,  see  Vega. 

Magnitudes,  scale  of,  257. 

Margaritifer  Sinus,  Martian  es- 
tuary, 154. 

Mars,  the  planet,  84,  216;  atmos- 
phere of,  147;  canals  of,  153, 
gemination  of,  156;  climate  of, 
147;  habitability  of,  524;  paral- 
lax of  (1672),  189,  191;  polar 
caps  of,  143;  Schiaparelli's  ob- 
servations of,  143;  spectrum  of, 
372;  topography  of,  150. 

Maupertuis,  the  "Earth-flattener," 
107. 

Maxwell,  Clerk,  magnetism  and 
light,  204;  investigations  of  Sat- 
urn's rings,  170. 

Measuring  machines,  294. 

Mercury,  the  planet,  84,  215: 
atmosphere  of,  138;  habitability 
of,  525;  libration  of,  136;  mo- 
tions of,  45;  observations  of 
Leverrier  and  Lilienthal,  133; 
Schiaparelli's  observations  of, 
133;  diurnal  parallax  of  (1677), 
190;  rotation  of,  135;  transit  of, 
190,  191. 

Meteor-streams,  181 ;  Schiaparel- 
li's theory,  292. 

Meteorites,  as  sources  of  life,  70 
note;  of  lunar  craters,  70;  of 


534 


INDEX 


the  sun's  heat,  44,  51;  increase 
of  earth's  mass  by  falling  of 
meteors,  64;  as  origin  of  stellar 
systems  (Darwin's  theories),  59, 
126,163,  173,  417;  spectra  of,  171, 
389;  nature  and  composition  of, 
170;  connection  of  comets  with, 
402,  463. 

Micrometer,  invention  of,  189. 

Milky  Way,  22,  277,  421,  437,  490. 

Mitchel,  O.  M.,  311. 

Mizar,  spectrum  of,  428. 

Moeris,  Lacus,  Martian  lake,  152. 

Molecular  theory,  5  et  seq.;  of 
solids,  8,  13;  of  gases,  10,  15, 
171 ;  of  liquids,  10,  12. 

Moon,  discoveries  of  Galileo  in 
regard  to  the,  342;  present  con- 
dition of  the,  78;  habitability  of 
the,  518;  light  and  heat  of  the, 
289;  photographs  of  the,  294; 
rotation  of  the,  66,  233;  spec- 
trum of  the,  371;  use  in  deter- 
mining parallax,  192. 

Motion  in  the  line  of  sight,  as 
determined  by  the  spectroscope, 
278,  423,  461,  484,  485. 

Motion,  proper,  272. 

Navigation,  early  American,  314. 

Nebula,  annular,  in  Lyra,  385, 
388;  dumb-bell,  385,  388;  in  An- 
dromeda, 169,  386,  417,  437,  494; 
in  Aquarius,  384;  in  Orion,  60, 
385,  427,  437,  491. 

Nebulae,  60,  167;  brightness  of  the, 
388;  first  photographs  of,  297; 
spectra  of,  170,  382,  415,  453. 

Nebular  hypothesis  of  Laplace, 
59,  125,  167,  170,  416,  492,  497. 

Neptune,  the  planet,  217;  discov- 
ery of,  263;  distance  from  the 
sun,  211 ;  habitability  of,  519; 
satellites  of,  habitability  of  the, 
524. 

Nerigos,  district  of  Mars,  151. 

Newton,  Sir  Isaac,  106,  347. 

Niliacus,  Lacus,  Martian  lake, 
151- 

Nilosyrtis,  canal  of  Mars,  153, 
155,  158. 

Noachis,  Martian  island,  154. 

Nodes,  lines  of,  229. 

Nutation,  192;  defined,  197;  use  in 

Observatories,  American,  early, 
319. 


Observatory,  work  in  a  modern, 
473- 

Olbers,  theory  of  the  origin  of 
asteroids,  249. 

Orion,  constellation  of,  258. 

Orion,  nebula  in,  60,  297,  300,  385, 
437,  49i;  motion  of,  427;  spec- 
trum of,  470. 

Orionis,  o,  see  Betelgeux. 

Orionis,  j8,  see  Rigel. 

Parallactic  inequality,  196. 

Parallax,  hypothetical,  286;  meth- 
ods of  determining,  192,  205, 
246,  326;  ancient  observations 
of,  188;  stellar,  321,  436,  279. 

Pegasi,  0,  spectrum  of,  375,  379. 

Peirce,   Benjamin,   315. 

Persei,   ft,  see  Algol. 

Persei,  T,  spectra  of,  431. 

Phoenicis,  Lacus,  Martian  lake, 
154- 

Photography,  celestial,  22,  165, 
242,  248,  290,  350,  431,  482. 

Photography,  spectroscopic,  391, 
426,  468. 

Photometry,  435;  at  Harvard  Uni- 
versity, 260,  298. 

Photo-tachymetry,  192,  200. 

Piazzi,  241. 

Planets,  analogies  between  the,  86; 
distances  of  the,  187;  habitabil- 
ity of  the,  83,  513;  magnitudes, 
etc.,  215;  orbits  of  the,  varia- 
tions in  the,  222;  mutual  per- 
turbations of  the,  200,  218; 
spectra  of  the,  371. 

Plumb-line,  deflection  of  the,  no. 
See  Theorem  of  Stokes. 

Poisson,  studies  of  terrestrial 
temperature,  116. 

Polar  caps,  of  Mars,  143. 

Pole,  celestial,  Barnard's  photo- 
graph of,  22;  changes  in  posi- 
tion of  the,  24. 

Pole,  terrestrial,  25;  see  North 
Pole. 

Poles,  magnetic,  changes  in,  how 
caused,  70. 

Pole,  North,  physical  and  astro- 
nomical conditions  at  the,  25; 
wanderings  of  the,  22. 

Pole  Star,  23,  25,  28. 

Pollux  (0  Geminorum),  proper 
motion  of,  279. 

Polynesians,  astronomy  of  the, 
256. 


INDEX 


535 


Precession,  192,  197;  use  in  deter- 
mining parallax,  197. 

Prominences,  solar,  465. 

Propontis,  Martian  lake,  152. 

Ptolemaic  system  of  astronomy, 
187. 

Ptolemy,  "Almagest"  of,  256, 
258. 

Pyrois,  151  note. 

Pyrrhse,  Regio, district  of  Mars,  151. 

Pythagoras,  astronomical  system 
of,  187. 

Refractors  and  reflectors,  347;  see 

Telescope. 

Regulus  (a  Leonis),  258. 
Rigel  (jSOrionis),  parallax  of,  492. 
Roche,    investigation    of    Saturn's 

rings,  170;  hypothesis  of,  as  to 

the  distribution   of  the  earth's 

mass,  114. 
Rutherfurd,  Lewis,  294. 

Sabaeus,  Sinus.     Martian  estuary, 

154- 

Sagittarii,  5,  spectrum  of,  431. 

Sappho,  the  asteroid,  246. 

Satellites,  distribution  of,  84;  ro- 
tation of  the,  141,  508;  stability 
of  satellite  systems,  232;  of  the 
great  planets,  habitability  of 
the,  524;  see  Moon. 

Saturn,  the  planet,  61,  216;  habita- 
bility of,  522;  photographs  of, 
301;  rings  of,  83,  170,  503,  dis- 
covery of  the,  345,  stability  of 
the,  234;  satellites  of,  84,  dis- 
covery of  the,  345,  habitability 
of  the,  524;  spectrum  of,  372. 

Schoenfeld,  "  Durchmusterung  " 
of,  265. 

Sextant,  invention  of  the,  310. 

Simois,  canal  of  Mars,  156. 

Sirenum,  Martian  sea,  150. 

Sirius  (o  Canoris  Majoris),  the 
Dog  Star,  256,  257;  distance  of, 
280;  proper  motion  of,  272; 
spectrum  of,  469. 

Snow,  melting  of,  140;  apparent 
existence  of,  on  Mars,  143. 

Solar  system,  magnitude  of  the, 
183;  origin  of,  59;  stability  of 
the,  213;  see  Nebular  Hypoth- 
esis, Sun,  Planets,  Satellites, 
etc. 

Solids,  molecular  theory  of,  8,  13. 

Solis,  Lacus,  Martian  sea,  150. 


Spectra  of  chemical  elements,  451 ; 
of  meteorites,  171;  of  nebulae, 
170;  of  the  stars,  see  Stars. 

Spectroscope,  278;  development  of 
the,  365,  391;  Muggins's  first 
stellar,  447. 

Spectroscopy,  celestial,  292,  363, 
391,  44i,  483- 

Spica  (o  Virginis),  spectrum  of, 
430,  462. 

Star  gauging,  284. 

Star  maps,  of  the  Berlin  Academy, 
262;  photographic,  299,  433,  482, 
490. 

Stars,  age  of  the,  406,  492;  binary, 
286,  spectroscopic  discovery  of, 
427,  462,  489;  catalogues  of,  269; 
colours  of  the,  376;  distances  of 
distribution  of  the,  282;  magni- 
tudes of,  257;  masses  of,  282; 
naming  of,  258;  number  of,  260; 
parallax  of  the,  324;  early  pho- 
tographs of  the,  296,  see  Celes- 
tial Photography;  spectra  of  the 
367,  406,  450,  484;  temporary, 
379,  457J  variable,  289,  378,  429. 

Stars,  shooting,  see  Meteorites. 

Stilbon,  ancient  name  for  Mer- 
cury, 134. 

Stokes,  theorem  of,  no. 

Struve,  observations  of  stellar 
parallax,  326,  331. 

Sun,  the,  chemical  composition  of, 
403;  distance  of,  211;  force  of 
gravity  at  surface  of,  17;  habi- 
tability of,  517;  heat  of,  I,  41, 
origin  and  total  amount  of,  50, 
sources  of,  47;  present  tem- 
perature of  the,  49;  Professor 
Young's  views  concerning  the 
present  condition  of,  67;  secular 
cooling  of,  44;  magnitude  of, 
215;  movement  of,  in  space, 
273;  photographs  of,  295;  rota- 
tion of,  431;  spectroscopic  ob- 
servations of,  465;  weight  of, 
176;  star  cluster  to  which,  be- 
longs, 287. 

Sun  spots,  290,  344. 

Swift's  comet,  22. 

Syrtis  Major,  Martian  gulf,  155. 

Tauri,  o,  see  Aldebaran. 

Telescope,  Galileo's,  189;  history 
of  the,  339;  mounting  of  the, 
353'  power  of  vision  attainable 
in  the,  516. 


536 


INDEX 


Tempe,  district  of  Mars,  149. 

Thule,  the  asteroid,  243. 

Tides,  39;  causes  of  the,  192;  effect 

of,  on  rotation,  289. 
Torricelli,    improvement    of    the 

telescope,  344. 
Transit  circle,  475. 
Triton,  a  canal  of  Mars,  156. 
Trivium  Charontis,  Martian  lake, 

152,  154- 
Tyrrhenum,  Martian  sea,  150. 

"  Uranometria  Nova,"  259;  "Ar- 
gentina," 259. 

Uranus,  the  planet,  217;  discovery 
of,  241,  349;  habitability  of,  519; 
satellites  of,  84,  habitability  of 
the,  524. 


Vega  (o  Lyrae),  25,  257;  parallax 
of,  280,  331;  proper  motion  of, 
279;  spectrum  of,  375,  469. 

Venus,  the  planet,  84,  216;  discov- 
ery of  the  phases  of,  by  Galileo, 
342;  habitability  of,  525;  paral- 
lax of  (1681),  190;  spectrum  of, 
372;  transits  of,  190,  191,  209, 
244. 

Victoria,  the  asteroid,  246. 

Virginis,  a,  see  Spica. 

Volcanoes,  124;  cause  of,  77. 

Whewell,  theory  of  life  in  other 
worlds,  84. 

Zodiacal  light,  181. 


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